Issue:
feat: Implement subscriptions workflow and incoming webhook support

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->

This pr adds support for the invoice-generated event type for the incoming webhook for Chargebee and creates a process tracker entry that will trigger a S2S call and invoice sync.

### 1. __Invoice Generated Webhook Support__

- Implements handling for Chargebee's `invoice_generated` webhook event
- Creates process tracker entries for subscription workflow management
- Added  SubscriptionId to  ObjectReferenceId for subscription webhook
- Created a get_mca_from_subscription_id function to fetch the MerchantConnectorAccount from the SubscriptionId

### 2. __Subscription Workflow Integration__

- Adds subscription workflow tracking data
- Implements process tracker integration for subscription events

### 3. __Database Changes__

- Insert details into the invoice table


### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
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Insert into subscription table
```
INSERT INTO public.subscription (
    id,
    status,
    billing_processor,
    payment_method_id,
    merchant_connector_id,
    client_secret,
    connector_subscription_id,
    merchant_id,
    customer_id,
    metadata,
    created_at,
    modified_at,
    profile_id,
    merchant_reference_id
) VALUES (
    '169vD0Uxi8JB746e',
    'active',
    'chargebee',
    'pm_98765',
    'mca_YH3XxYGMv6IbKD0icBie',
    'secret_subs_1231abcd',
    '169vD0Uxi8JB746e',
    'merchant_1758710612',
    'gaurav_test',
    '{"plan":"basic","trial":true}',
    NOW(),
    NOW(),
    'pro_RkLiGcSuaRjmxA5suSfV',
    'ref_subs_1231abcd'
);
```
incoming webhook for chargebee
```
curl --location 'http://localhost:8080/webhooks/merchant_1758710612/mca_YH3XxYGMv6IbKD0icBie' \
--header 'api-key: dev_5LNrrMNqyyFle8hvMssLob9pFq6OIEh0Vl3DqZttFXiDe7wrrScUePEYrIiYfSKT' \
--header 'Content-Type: application/json' \
--header 'authorization: Basic aHlwZXJzd2l0Y2g6aHlwZXJzd2l0Y2g=' \
--data-raw '{
    "api_version": "v2",
    "content": {
        "invoice": {
            "adjustment_credit_notes": [],
            "amount_adjusted": 0,
            "amount_due": 14100,
            "amount_paid": 0,
            "amount_to_collect": 14100,
            "applied_credits": [],
            "base_currency_code": "INR",
            "channel": "web",
            "credits_applied": 0,
            "currency_code": "INR",
            "customer_id": "gaurav_test",
            "date": 1758711043,
            "deleted": false,
            "due_date": 1758711043,
            "dunning_attempts": [],
            "exchange_rate": 1.0,
            "first_invoice": false,
            "generated_at": 1758711043,
            "has_advance_charges": false,
            "id": "3",
            "is_gifted": false,
            "issued_credit_notes": [],
            "line_items": [
                {
                    "amount": 14100,
                    "customer_id": "gaurav_test",
                    "date_from": 1758711043,
                    "date_to": 1761303043,
                    "description": "Enterprise Suite Monthly",
                    "discount_amount": 0,
                    "entity_id": "cbdemo_enterprise-suite-monthly",
                    "entity_type": "plan_item_price",
                    "id": "li_169vD0Uxi8JBp46g",
                    "is_taxed": false,
                    "item_level_discount_amount": 0,
                    "object": "line_item",
                    "pricing_model": "flat_fee",
                    "quantity": 1,
                    "subscription_id": "169vD0Uxi8JB746e",
                    "tax_amount": 0,
                    "tax_exempt_reason": "tax_not_configured",
                    "unit_amount": 14100
                }
            ],
            "linked_orders": [],
            "linked_payments": [],
            "net_term_days": 0,
            "new_sales_amount": 14100,
            "object": "invoice",
            "price_type": "tax_exclusive",
            "recurring": true,
            "reference_transactions": [],
            "resource_version": 1758711043846,
            "round_off_amount": 0,
            "site_details_at_creation": {
                "timezone": "Asia/Calcutta"
            },
            "status": "payment_due",
            "sub_total": 14100,
            "subscription_id": "169vD0Uxi8JB746e",
            "tax": 0,
            "term_finalized": true,
            "total": 14100,
            "updated_at": 1758711043,
            "write_off_amount": 0
        }
    },
    "event_type": "invoice_generated",
    "id": "ev_169vD0Uxi8JDf46i",
    "object": "event",
    "occurred_at": 1758711043,
    "source": "admin_console",
    "user": "ankit.gupta.001@juspay.in",
    "webhook_status": "scheduled",
    "webhooks": [
        {
            "id": "whv2_169mYOUxi3nvkZ8v",
            "object": "webhook",
            "webhook_status": "scheduled"
        }
    ]
}'
```

response

200 ok 

<img width="2551" height="555" alt="image" src="https://github.com/user-attachments/assets/29c9f075-cbb6-4a3d-ace0-cee8a884644a" />

db entry into invoice table

<img width="578" height="449" alt="image" src="https://github.com/user-attachments/assets/23614dcf-70fc-480b-9922-0d5645e1d333" />

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Resolution:
<think>
PR Context:
- Title: refactor(webhooks): Fix incoming webhooks failure
- Merged: 2025-10-16T19:20:19Z
- Files changed: 2 files
- Commits: 1 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->

This pr fixes bug due to Webhooks PR [#9400](https://github.com/juspay/hyperswitch/pull/9400), causing all incoming webhooks to fail for merchant 
### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the
</think>

To implement this, you'll need to modify:


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