Add the readiness probe vocabulary for services: tcp, http, log

[ishandhanani]

Stacked on #400 (Track 2 step 8 of #385). Draft until the stack below it merges.

What

services[].readiness gains the probe vocabulary from the schema design: exactly one of

readiness:
  port: 9000                 # shorthand for tcp

readiness:
  tcp:
    port: 9000               # a TCP connection is accepted

readiness:
  http:
    port: 8000
    path: /ready             # GET http://<node>:8000/ready
    status: 200              # returns this status

readiness:
  log:
    pattern: 'Uvicorn running on .*:\d+'   # regex against service_<name>.out

plus timeout_seconds (default 120) and interval_seconds (default 2). The generic wait loop in core/readiness.py re-runs the probe until it passes, the deadline expires, or the process dies, so a crashed service fails the job at once with its exit code instead of after the full timeout. The port-collision check now keys on the port a tcp or http probe implies.

Per the design, the global health_check block and the worker and frontend health checks are untouched: this fixes the silent-dead-sidecar class (NVIDIA/InferenceMAX#271) without touching the downstream recipes that set health_check.

Also

• examples/features/services.yaml now gates on an http probe against the log browser's directory listing.
• tests/test_readiness.py covers the schema (shorthand, exactly-one, invalid regex/path/interval), each probe, and the wait loop with a fake clock (retries, fail-fast on death, clipped final sleep). tests/test_services.py updated for the probe API.
• Docs: probe section in docs/services.md, config-reference row, regenerated schema reference.

Validation

• Full suite green (1835 passed); lint, schema-docs drift check, every example validates.
• sa-b200, job 12295: the services example gated on http://<node>:9911/ -> 200 came ready after 40s, the benchmark completed, and the job finished clean (3m25s).

[ishandhanani]

Consolidated into #407, which carries these same commits as one PR against main (this description is reproduced there as one of its parts). Closing to keep review in one place; the branch stays for per-step reference via stack #398.