feat: add session.getMetadata to all SDK languages

[MackinnonBuck]

Motivation

The runtime now exposes a session.getMetadata JSON-RPC method (copilot-agent-runtime#5021) that provides efficient O(1) lookup of a single session's metadata by ID — avoiding the need to call session.list and filter client-side. This PR exposes that endpoint in all four SDK language bindings.

Approach

Each SDK already has listSessions returning SessionMetadata[]. The new method reuses the same SessionMetadata type (no new public types needed) but returns a single optional result. When the session is not found, the method returns the language-idiomatic "absent" value rather than throwing an error.

SDK	Method	Return type	Not-found value
Node.js	getSessionMetadata(sessionId)	Promise<SessionMetadata | undefined>	undefined
Python	get_session_metadata(session_id)	SessionMetadata | None	None
Go	GetSessionMetadata(ctx, sessionID)	(*SessionMetadata, error)	nil
.NET	GetSessionMetadataAsync(sessionId, ct)	Task<SessionMetadata?>	null

Changes

• Node.js — New method in client.ts with full JSDoc; skipped E2E test (matching existing listSessions skip pattern)
• Python — New method in client.py with docstring; running E2E test (Python/Go session lifecycle tests are live)
• Go — New method in client.go, internal request/response structs in types.go; running E2E test
• .NET — New method in Client.cs, internal records + [JsonSerializable] registration; skipped E2E test
• Snapshot — test/snapshots/session/should_get_session_metadata.yaml for the replay proxy

Notes

• E2E tests in Node.js and .NET are skipped with the same TODO as the existing listSessions tests ("Re-enable once test harness CAPI proxy supports this test's session lifecycle"). Python and Go E2E tests are running, consistent with their existing session lifecycle tests.
• All E2E tests cover both the happy path (create session → get metadata → verify fields) and the not-found case.

[github-actions]

✅ Cross-SDK Consistency Review

I've reviewed this PR for consistency across all four SDK implementations (Node.js, Python, Go, and .NET). This PR maintains excellent cross-language consistency!

Summary

This PR successfully adds the session.getMetadata RPC method to all four SDK languages with consistent:

✅ API Design

• Node.js: getSessionMetadata(sessionId) → Promise(SessionMetadata | undefined)
• Python: get_session_metadata(session_id) → SessionMetadata | None
• Go: GetSessionMetadata(ctx, sessionID) → (*SessionMetadata, error)
• .NET: GetSessionMetadataAsync(sessionId, ct) → Task(SessionMetadata?)

All follow language-specific naming conventions (camelCase, snake_case, PascalCase) and return idiomatic "not found" values (undefined, None, nil, null).

✅ Implementation Pattern

• All SDKs reuse the existing SessionMetadata type (no new types needed)
• All call the same JSON-RPC method: session.getMetadata
• All properly check client connection state before making the call
• All handle the not-found case gracefully without throwing errors

✅ Documentation

• Consistent descriptions across all languages emphasizing O(1) efficiency
• Similar code examples demonstrating usage
• Proper parameter and return type documentation

✅ Test Coverage

• Node.js and .NET: Tests present but skipped (matching existing listSessions pattern with same TODO)
• Python and Go: Active E2E tests (consistent with their existing session lifecycle test patterns)
• All tests cover both success and not-found cases
• Shared snapshot file: test/snapshots/session/should_get_session_metadata.yaml

No Consistency Issues Found

This PR maintains feature parity and consistent API design across all SDK implementations. Great work! 🎉

Generated by SDK Consistency Review Agent for issue #899 · ◷