fix: gRPC metadata header casing and invocation_metadata() call

[guglielmo-san]

This PR fixes two issues related to the grpc support:

1. The extension headers key in the metadata are supposed to be lower key (ref)
2. Invokes correctly the servicer_context.invocation_metadata as a method instead of as a property

For each of these issues was respectively opened a community PR (#635 , #673). To speed up the integration of these changes, this new PR has been created to integrate both changes at the same time.

Fixes #656

[gemini-code-assist]

Summary of Changes

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This pull request addresses two critical issues in the gRPC support: ensuring that extension header keys are always lowercase for compliance with gRPC standards and correcting the invocation of the invocation_metadata method on ServicerContext. These changes improve the robustness and correctness of gRPC communication within the system, resolving potential interoperability problems and API misuse.

Highlights

• gRPC Extension Header Standardization: Standardized the gRPC extension header keys to be consistently lowercase, aligning with gRPC specifications for metadata handling.
• Correct invocation_metadata Usage: Updated the usage of grpc.aio.ServicerContext.invocation_metadata to correctly invoke it as a method rather than accessing it as a property.
• Consolidated Fixes: This pull request integrates two previously identified issues and their respective community PRs (fix: Update invocation metadata from grpc.aio.ServicerContext to use the correct syntax #635, fix(grpc): normalize extension metadata header key to lowercase #673) into a single, comprehensive fix.

Changelog

• src/a2a/client/transports/grpc.py
  • Ensured HTTP_EXTENSION_HEADER is converted to lowercase when constructing gRPC metadata.
  • Refactored logic for determining which extensions to use for metadata creation.
• src/a2a/server/request_handlers/grpc_handler.py
  • Changed context.invocation_metadata to context.invocation_metadata() to correctly call it as a method.
  • Applied .lower() to HTTP_EXTENSION_HEADER when retrieving and setting metadata to ensure lowercase keys.
  • Adjusted the placement of lower_key = key.lower() for improved consistency.
• tests/client/transports/test_grpc_client.py
  • Updated test assertions to reflect the change in HTTP_EXTENSION_HEADER to its lowercase form in metadata.
• tests/integration/test_client_server_integration.py
  • Added an extensions parameter to transport.send_message calls in integration tests.
  • Included an assertion to verify that received_context.requested_extensions correctly captures the sent extensions.
• tests/server/request_handlers/test_grpc_handler.py
  • Modified mock invocation_metadata to be a mock method returning grpc.aio.Metadata.
  • Updated mock metadata creation and assertions to use HTTP_EXTENSION_HEADER.lower().

Activity

• No human activity has occurred on this pull request yet.

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[gemini-code-assist]

Code Review

This PR addresses two issues in the gRPC support: standardizing extension headers to lowercase and correcting the invocation of servicer_context.invocation_metadata as a method. The changes involve modifying the client and server-side code to ensure consistency in header casing and proper method invocation, along with updating the corresponding tests to reflect these changes.