Issue: feat(core): Add kill switch for Unified Connector Service (UCS)

# feat(core): Add kill switch for Unified Connector Service (UCS)

## Feature Description

We need to implement a kill switch mechanism for the Unified Connector Service (UCS) integration that allows operators to disable UCS immediately without requiring code changes or deployments.

## Problem Statement

Currently, if UCS experiences issues or needs to be disabled for operational reasons, there's no quick way to fall back to traditional connector flows without modifying code and deploying. This poses operational risks during incidents.

## Proposed Solution

Implement a database-driven kill switch that:
- Can be toggled via a configuration entry in the database
- Takes effect immediately without requiring service restart
- Integrates seamlessly with the existing UCS decision flow
- Provides clear logging when the kill switch is active

## Technical Approach

1. Add a new configuration key `UCS_KILL_SWITCH_ACTIVE` that can be stored in the database
2. Implement a helper function to check the kill switch status
3. Integrate the kill switch check into the UCS decision flow before rollout percentage checks
4. Ensure proper error handling when UCS is disabled via kill switch

## Benefits

- **Operational Control**: Immediate ability to disable UCS during incidents
- **Risk Mitigation**: Quick fallback mechanism without deployment delays
- **Debugging**: Easier troubleshooting by toggling UCS on/off
- **Gradual Rollout**: Additional safety layer on top of percentage-based rollout

## Implementation Details

The kill switch will be checked in the `should_call_unified_connector_service` function before any other conditions, ensuring it has the highest priority in the decision flow.

## Acceptance Criteria

- [ ] Kill switch configuration can be set in the database
- [ ] When kill switch is active, UCS is bypassed completely
- [ ] Changes take effect immediately without service restart
- [ ] Proper logging indicates when kill switch is active
- [ ] Traditional connector flow continues to work when UCS is disabled

Resolution: <think>
PR Context:
- Title: feat(core): Implement UCS kill switch for emergency fallback
- Merged: 2025-07-22T09:00:55Z
- Files changed: 4 files
- Commits: 8 commits

Implementation approach:
# feat(core): Implement UCS enable/disable configuration for operational control

## Type of Change
  <!-- Put an `x` in the boxes that apply -->

  - [ ] Bugfix
  - [x] New feature
  - [ ] Enhancement
  - [ ] Refactoring
  - [ ] Dependency updates
  - [ ] Documentation
  - [ ] CI/CD

  ### Fixes #8644 

  ## Description
  <!-- Describe your changes in detail -->

  This PR implements a **database-driven enable/disable configuration** for the Unified Connector Service (UCS) int
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(core): Implement UCS kill switch for emergency fallback (#8651)
Files to modify:


**crates/router/src/consts.rs**
  Modify existing code

**crates/router/src/core/payments.rs**
  Modify existing code

**crates/router/src/core/payments/helpers.rs**
  Add:
    - function: pub::is_ucs_enabled

**crates/router/src/core/unified_connector_service.rs**
  Modify existing code