feat(ext): add /api/metrics fallback and single-flight startup guard

[bertheto]

Summary

• /api/metrics fallback probe: When /health is unavailable during startup (slow backend, Uvicorn reloader delay, or legacy Python server), the extension now falls back to /api/metrics before declaring the server as down. This eliminates false-negative startup failures observed in practice (follow-up to fix(server): UP-33 - Default AGENTCHATBUS_RELOAD to 0 and pass it explicitly from extension #61).
• Single-flight startup guard: ensureServerRunning() is now wrapped in a createSingleFlightRunner() to coalesce concurrent calls from VS Code activation and MCP provider resolution, preventing duplicate startup sequences.
• Structured probe diagnostics: Probe failures now produce typed, descriptive messages (timeout duration, HTTP status, network error) logged during both initial probe and the wait-for-ready loop, improving startup troubleshooting.

Changes

File	What
src/logic/busServerManager.ts	New types (MetricsPayload, StartupProbeEndpoint, StartupProbeOutcome, StartupProbeResolution) + helpers (describeStartupProbeFailure, resolveStartupProbeResult, createSingleFlightRunner)
src/busServerManager.ts	Generic probeJsonEndpoint, dual-endpoint probeServer(), single-flight wrapper, timeout 1s to 2s, failure logging
src/logic/testExports.ts	3 new re-exports
test/logicBusServerManager.test.js	4 new tests (probe failure description, probe resolution with /health preference, /metrics fallback, single-flight coalescing)

What is NOT changed

• All workspace-dev-service logic remains intact and unmodified
• No changes to workspaceDev.ts or workspace-dev launch specs
• Existing tests continue to pass unchanged

Test plan

• npm run compile passes (TypeScript clean)
• node --test test/logicBusServerManager.test.js: 12/12 pass (7 existing + 4 new + 1 async)
• node --test test/logicWorkspaceDev.test.js: 2/2 pass (no regression)
• Manual verification: install the built .vsix and confirm startup succeeds when /health is slow but /api/metrics responds