Issue: Migrate from cargo2nix to rust-flake

Migrate the Nix build/shell system from cargo2nix to rust-flake.

**Parent Issue:** #57  
**Related PR:** #8503

## Acceptance Criteria
- [x] Migrate from cargo2nix to rust-flake
- [x] Verify nix CI/CD compatibility


Resolution: <think>
PR Context:
- Title: refactor(nix): Migrate from cargo2nix to rust-flake
- Merged: 2025-07-15T08:20:23Z
- Files changed: 7 files
- Commits: 6 commits

Implementation approach:
# Migrate Nix Configuration to rust-flake

This PR refactors the existing Nix setup by migrating from `cargo2nix` to `rust-flake`.
Resolves: #8538 

## TODO:

- [x] **Initial port** - Port current `flake.nix` functionality using rust-flake
  - [x] @kashif-m The current [`flake.nix`](https://github.com/juspay/hyperswitch/blob/d305fad2e6c403fde11b9eea785fddefbf3477df/flake.nix#L63-L78) has `default`, `dev`, and `qa` shells. Are all three required? Typically we use `default` for development
</think>

To implement this, you'll need to modify:


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