docs: add @param and @returns TSDoc to a2a utility functions

[nuthalapativarun]

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Link to Issue or Description of Change

Adds @param and @returns (and @throws where applicable) TSDoc to all exported functions across the four A2A utility files that had only one-line descriptions:

a2a_remote_agent_utils.ts

• getUserFunctionCallAt — adds @param session, @param index, @returns
• isFunctionCallEvent — adds @param event, @param callId, @returns
• getFunctionResponseCallId — adds @param event, @returns
• toMissingRemoteSessionParts — adds @param ctx, @param session, @returns
• presentAsUserMessage — adds @param ctx, @param agentEvent, @returns

metadata_converter_utils.ts

• getAdkEventMetadata — adds @param a2aEvent, @returns
• getA2AEventMetadata — adds @param adkEvent, @param appName/userId/sessionId, @returns
• getA2ASessionMetadata — adds @param appName/userId/sessionId, @returns
• getA2AEventMetadataFromActions — adds @param actions, @returns

part_converter_utils.ts

• toA2AParts, toA2APart, toA2ATextPart, toA2AFilePart, toA2ADataPart — adds params and returns
• toGenAIContent — adds missing JSDoc entirely
• toGenAIParts, toGenAIPart, toGenAIPartText, toGenAIPartFile, toGenAIPartData — adds params and returns

event_converter_utils.ts

• toA2AMessage — adds @param event/appName/userId/sessionId, @returns
• toAdkEvent — adds @param event/invocationId/agentName, @returns

Testing Plan

Unit Tests:

• All unit tests pass locally.

Test Files: 13 passed (13)
     Tests: 180 passed (180)

Manual End-to-End (E2E) Tests:
Documentation-only change; no runtime behavior is altered. Verified with npm run build, npm run lint, and npm test — all pass.

Checklist

• Read CONTRIBUTING.md
• Self-reviewed code
• Commented hard-to-understand areas
• Added tests proving fix/feature works
• Unit tests pass locally
• Manually tested end-to-end
• Dependent changes merged/published

[nuthalapativarun]

Closing in favour of the consolidated PR #413, as requested by @kalenkevich. All changes from this PR are included there.