Add integration tests and align proto with RFC spec

[ajit-zer07]

Summary

• Add three-tier integration test suite exercising the runtime through the real gRPC boundary
  • Tier 1 (47 tests): scripted protocol tests for all modes, error paths, signals, version binding, dedup, cancel auth, discovery, streaming
  • Tier 2 (5 tests): Rig agent framework tools calling MACP operations via ToolSet::call()
  • Tier 3 (3 tests, #[ignore]): real OpenAI GPT-4o-mini agents coordinating through the runtime
• Align proto/macp/v1/core.proto with RFC spec — zero diff between runtime and normative proto
• Remove non-normative admin RPCs (GetRuntimeMetrics, GetSessionHistory) and Prometheus HTTP endpoint from runtime; these are served by the control plane instead
• Add ParticipantActivity message and enrich SessionMetadata with participant list and activity tracking (promoted to RFC spec)
• Enable ProgressCapability and WatchSignals ambient signal broadcast
• Add participant activity tracking (per-sender message counts and last-seen timestamps) to sessions
• Remove unused hyper/hyper-util/http-body-util dependencies

Proto changes (normative)

Added to RFC spec core.proto:

• ParticipantActivity { participant_id, last_message_at_unix_ms, message_count }
• SessionMetadata fields 9-10: participants, participant_activity

Removed from runtime core.proto (moved to control plane):

• GetRuntimeMetricsRequest/Response, ModeMetricsEntry
• GetSessionHistoryRequest/Response, HistoryEntry
• GetRuntimeMetrics and GetSessionHistory RPCs from MACPRuntimeService

Test plan

• cargo test — 301 unit/conformance tests pass
• cargo clippy — no warnings
• cargo build — clean build, no hyper/http-body-util deps
• diff proto/macp/v1/core.proto ../multiagentcoordinationprotocol/schemas/proto/macp/v1/core.proto — zero differences
• Integration tests: MACP_TEST_BINARY=../target/debug/macp-runtime cargo test -- --test-threads=1 — 52 tests pass (47 Tier 1 + 5 Tier 2)
• Tier 3 E2E tests require OPENAI_API_KEY
• Control plane npm run build — clean build