fix: make MCP startup readiness provider neutral [risk:medium]

[Leeeon233]

Risk: 🟡 medium | Confidence: high — changes MCP session-open ordering and thread-scoped startup lifecycle; covered by the full adapter test suite

Summary

• wait for every client-requested MCP server to reach a terminal Codex startup state before new, load, resume, or fork completes
• publish failed and cancelled startup updates synchronously; for load, emit them after history replay and before the response
• isolate startup status and waiters by Codex thread, with cleanup on thread and connection close
• preserve global/project MCP configuration merging without client-specific metadata or server-name assumptions
• document the provider-neutral startup-readiness invariant in the adapter context

Verification

• corepack pnpm test — 381 passed, 29 skipped
• corepack pnpm run typecheck
• corepack pnpm run build

Review focus

• notification ordering around load-session history replay
• thread-keyed waiter cleanup during concurrent session opens and connection shutdown
• compatibility with Codex global/project MCP configuration merging