[YuqiGuo105]

Link to Issue or Description of Change

Closes #1438.

Problem:
RemoteA2AAgent has no application hook for populating an outgoing A2A message's metadata. Applications cannot explicitly pass selected context, such as a user or tenant identifier, without mixing it into conversation content.

Solution:

• Add the optional A2ARequestMetadataProvider callback and RemoteA2AAgent.Builder.requestMetadataProvider(...).
• Invoke the provider for each outgoing message after its parts and identifiers have been populated, including function-call continuations.
• Attach a shallow copy of the returned map to Message.metadata. Preserve existing behavior when no provider is configured or the provider returns null; stop the send if the provider throws.
• Do not forward session state automatically or impose a fixed set of metadata keys.

Example builder configuration:

.requestMetadataProvider((context, message) ->
    Map.of("user_id", context.session().state().get("user_id")))

Testing Plan

Unit Tests:

• Added 12 regression tests covering opt-in selection, default/null/empty behavior, provider reevaluation, message/task/context preservation, defensive copying, callback failure, and an empty session.
• Verified metadata survives the SDK models' JSON serialization/deserialization and remains distinct from request-level metadata.
• Verified an ADK receiver can read message metadata in its beforeExecuteCallback, while existing request metadata propagation into RunConfig is unchanged.
• Ran ./mvnw -o -q -pl a2a -am test: the full core and A2A module test suites pass, including all 151 A2A tests.
• Ran git diff --check and applied the repository's Google Java formatting.

Manual End-to-End (E2E) Tests:

Not run against a deployed remote agent. The tests exercise the sender, the SDK's JSON models, and the receiver callback locally; they do not claim transport-level E2E coverage. No model credentials or external services are needed for these tests.

Checklist

• I have read CONTRIBUTING.md.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• Added concise Javadocs for the callback and its semantics.
• New and existing unit tests pass locally.
• I have manually tested my changes end-to-end.

Additional context

This implements message-level metadata, as requested by #1438. On an ADK receiver it is readable via RequestContext.getMessage().getMetadata(). It is not automatically copied into session state or RunConfig.customMetadata(); the latter's existing a2a_metadata path consumes request-envelope metadata instead.

The current Python a2a_request_meta_provider populates request-level metadata. This PR does not claim identical wire semantics: request-envelope metadata and transport authentication headers are intentionally out of scope here.