test(sessions): synchronize worker readiness in proxy flow

[YueZh127]

Purpose

Fix the deterministic readiness race exposed twice by PR #37 CI. createSession returns submitted; the proxy integration test now waits for the worker to complete its claim and begin executor creation before sending the first action. Production session semantics are unchanged.

Behavior

• Uses an explicit executor-ready signal, not sleeps or retrying 409.
• Races readiness against worker failure, early stop, and a bounded diagnostic deadline.
• Verifies claimed|running through the same NyxID-style proxy.
• Preserves raw response bodies in HTTP failure diagnostics.

Verification

• Focused proxy test: pass.
• Concurrent local runs: 12/12 pass.
• Independent reviewer repeated 20 runs: 40/40 cases pass.
• Full suite: 53 files / 370 tests pass.
• Standalone Mongo contract: 80/80 pass.
• Build, typecheck, lint, bundle, fixture drift, and both contract demos pass.
• Independent review: initial P2/P3 findings fixed; same reviewer approved the final diff.

Normal merge commit into staging only. No squash, rebase, force-push, auto-merge, or branch deletion.