feat: ccp-5627-remove-gc-notify-passthrough

[revanth-banala]

Description

Removes the legacy /gcnotify-passthrough/v2/... API while preserving the native GC Notify-compatible /gcnotify/v2/... routes and their upstream fallback behavior.
Changes

• Deleted GcNotifyPassthroughController and its tests.
• Removed passthrough controller and ApiKeyGuard module registration.
• Removed the passthrough path from the NestJS global-prefix exclusions.
• Removed all seven passthrough routes from the Kong gateway configuration.
• Retained GcNotifyApiClient for native-route fallback and bulk sends.
• Retained the ApiKeyGuard implementation and tests on disk.
• Updated OpenAPI documentation to describe native GC Notify-compatible endpoints.
• Rewrote GC Notify README documentation around tenant-aware internal execution and upstream fallback.

Fixes # (issue)

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• New unit tests
• New integrated tests
• New component tests
• New end-to-end tests
• New user flow tests
• No new tests are required
• Manual tests (description below)
• Updated existing tests

Checklist

• I have read the CONTRIBUTING doc
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have already been accepted and merged

Further comments

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