fix: support both api-key header formats for V2 authentication

[ItsYash1421]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

Fixed V2 customer creation authentication to accept API keys in both the documented OpenAPI format and the legacy format for backward compatibility.

Changes:

• Modified V2ApiKeyAuth::authenticate_and_fetch() to accept API keys from both api-key header and Authorization: api-key= header.
• Updated OpenAPI spec security scheme description to document both formats.
• Added authentication guidance to V2 customer create endpoint documentation.
• Added 6 comprehensive unit tests to verify both header formats work correctly.

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

Fixes #10789
Problem: Users following the OpenAPI documentation received authentication errors when attempting to create V2 customers:

• OpenAPI spec documented: api-key: header.
• Code expected: Authorization: api-key= header.
• Error received: {"error":{"type":"invalid_request","message":"Missing required param: Authorization","code":"IR_04"}}.

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
api-reference/v2/openapi_spec_v2.json	37% smaller
crates/router/src/services/authentication.rs	8% smaller
crates/openapi/src/routes/customers.rs	0% smaller

[SanchithHegde]

Hey @ItsYash1421, thanks for the PR. Unfortunately, we would most likely not be proceeding with this PR, as we would prefer to stick to the existing authentication mechanism for the v2 APIs (as I understand, maybe @AnuthaDev can confirm). That said, @AnuthaDev has fixed the incorrect API docs in #10888.

Thanks again for your time, feel free to pick up another issue if you're interested to contribute!

[SanchithHegde]

Closing this in favor of #10888.