Make the mock server faithful to the backend: rate-limit headers + no_data 404s

[MarketDataDev03]

Make the mock server faithful to the backend: rate-limit headers + no_data 404s

Summary

Two divergences between examples/mock-server and the real backend (../api/)
were surfaced in review. Both are fixed here so consumer demos exercise the same
wire behavior a real consumer sees.

1. Rate-limit headers were never emitted. The mock only set cf-ray and
   Content-Type, so client.getRateLimits() always returned null against it
   and the §10.3 exhausted-credits preflight path could not be exercised without
   hand-scripting all four headers on every step. The real backend attaches
   X-Api-Ratelimit-{Limit,Remaining,Reset,Consumed} to every response via
   update_user_quota (api/.../common/util/user_quotas_helper.py:162).

2. Unknown routes returned the wrong envelope. The mock answered
   404 {"s":"error","errmsg":"..."}. The backend's custom_404
   (api/.../marketDataApi/views.py:50) returns 404 {"s":"no_data"}. This
   matters because the SDK parses {"s":"error"} into a thrown ParseError
   while {"s":"no_data"} is a successful empty response — so the mock made a
   no-data path look like a failure.

Changes

examples/mock-server/server.py

• Add _default_ratelimit_headers() — emits all four x-api-ratelimit-*
  headers (all-or-nothing, matching RateLimitHeaders.parse), reset 24h out.
• Inject them on the default response path and on scripted responses
  (via setdefault, so a step can override e.g. x-api-ratelimit-remaining: 0).
• Unknown routes now return 404 {"s":"no_data"} (and inherit the rate-limit
  headers, just like the backend's custom_404).

examples/mock-server/README.md

• Update the defaults table (404 → no_data) and document the rate-limit
  headers + how to script the exhausted-credits case.

Exhausted-credits scenario

The headers are scriptable, so driving the preflight is one step:

{"steps": [{"status": 200, "body": "{\"s\":\"ok\"}",
            "headers": {"x-api-ratelimit-remaining": "0",
                        "x-api-ratelimit-reset": "9999999999"}}]}

The step's remaining: 0 wins; the other three headers auto-fill.

Verification

Ran the server (uvicorn server:app) and hit it with curl:

• GET /status/ (default) → 200 with all four x-api-ratelimit-* headers
  present → getRateLimits() now returns a real snapshot instead of null.
• GET /v1/stocks/quotes/AAPL/ (unknown route) → 404 {"s":"no_data"} with
  rate-limit headers attached.
• Script with x-api-ratelimit-remaining: 0 → step value wins, other three
  headers auto-fill.
• python -m py_compile server.py → clean.

Regression check

• Every {"s":"error"} body in examples/consumer-test/ is scripted explicitly
  via Step.of(...); none relied on the default 404 envelope, so the switch to
  no_data does not affect them.
• ResponseFeaturesApp already scripted its own 404 {"s":"no_data"}, which is
  now consistent with the default.

Scope

Mock-server / examples only. No changes to the SDK's src/main or its JUnit
suite (the CapturingClient stub already covers these scenarios at the wire
level).

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