Add headers support for MongoDB Atlas API v2 compatibility

[Copilot]

This PR adds support for custom headers in the MongoDB Atlas API client to enable compatibility with the new MongoDB Atlas Administration API v2.

Progress Checklist

• Add headers support to HttpClient constructor and methods
• Update entry point to pass headers from configuration to HttpClient
• Add TypeScript interface definition for headers
• Add comprehensive test suite for headers functionality
• Update existing tests to include headers parameter
• Add header verification tests for AtlasSearch methods
• Ensure all tests pass with 100% code coverage
• Address code review feedback

Recent Changes

Added specific tests to verify that custom headers are properly passed through when using AtlasSearch methods:

• Integration test showing headers are included in actual HTTP requests
• Unit test verifying headers are passed to underlying request calls

Implementation Details

• HttpClient: Modified constructor to accept optional headers parameter and updated fetch/fetchStream methods to properly merge default headers with method-specific headers
• Entry Point: Updated getMongodbAtlasApiClient to pass headers from configuration to HttpClient
• TypeScript: Added headers?: Record<string, string> to AtlasClientConfig interface
• Header Merging: Custom headers are merged with method-specific headers (like Content-Type) and httpOptions headers, with method-specific headers taking precedence

Testing

Added comprehensive test suite covering:

• Default header inclusion in requests
• Header merging with method-specific headers
• Header merging with httpOptions headers
• Backward compatibility (works without headers parameter)
• Edge cases with empty headers
• AtlasSearch-specific header verification

All tests pass with 100% code coverage maintained.

Fixes #58.

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[montumodi]

@copilot shouldn't there be a check that headers are being passed correctly?

[Copilot]

You're absolutely right! I've added tests to verify that headers are being passed correctly when using AtlasSearch methods:

1. Integration test: Verifies that custom headers configured on the client are included in actual HTTP requests made by AtlasSearch methods
2. Unit test: Verifies that custom headers are properly passed to the underlying request mechanism

The tests ensure that when a client is configured with custom headers (like "Accept": "application/vnd.atlas.2025-03-12+json"), those headers are correctly propagated through AtlasSearch method calls.

Commit: 1105829