fix: Add missing metadata field to Part message in gRPC specification

[ipapapa]

Summary

• Adds missing metadata field to the Part message in the gRPC specification
• Fixes inconsistency between TypeScript/JSON and gRPC protocol definitions
• Prevents data loss during Python ↔ gRPC conversions

Problem

The Part message in specification/grpc/a2a.proto was missing the metadata field that exists in the TypeScript and Python specifications. This caused important metadata (like ADK function call/response types) to be lost during conversions between Python and gRPC representations.

Solution

Added google.protobuf.Struct metadata = 4; to the Part message, following the same pattern used by other proto messages in the specification (Task, Message, Artifact, etc.).

Fixes #1005

[gemini-code-assist]

Summary of Changes

Hello @ipapapa, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical issue where important metadata was being lost during data conversions between Python and gRPC due to a missing field in the gRPC specification's Part message. By adding the metadata field, the change ensures data consistency across different language specifications and prevents data integrity problems, particularly for information like ADK function call/response types.

Highlights

• gRPC Specification Update: Adds the previously missing "metadata" field to the "Part" message within the gRPC specification.
• Cross-Language Consistency: Resolves an inconsistency between TypeScript/JSON and gRPC protocol definitions, ensuring uniform data structures.
• Data Integrity: Prevents potential data loss during conversions between Python and gRPC representations by ensuring all relevant metadata is preserved.

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[gemini-code-assist]

Code Review

This pull request correctly addresses an inconsistency in the gRPC specification by adding the metadata field to the Part message. The change is well-justified, as it prevents data loss during conversions and aligns the gRPC protocol definition with the existing TypeScript, JSON, and documentation specifications. The implementation is clean, uses the correct field type and tag, and follows the conventions of the existing protobuf file. This is a solid fix.