Studio: add option to disable the in-memory API monitor

[gaurav0107]

Summary

Studio records every OpenAI-compatible API request into an in-memory
ApiMonitor (the "API Monitor" activity view). A user running Studio purely as
an inference API, with logging/telemetry handled by an external gateway (e.g.
LiteLLM on a separate host), asked for a way to turn this off (#7036).

This adds an opt-in UNSLOTH_STUDIO_DISABLE_API_MONITOR environment variable,
parsed with the same truthy set ({"1","true","yes","on"}) the backend already
uses elsewhere (transformers_latest.py, llama_admission.py, ...). When set:

• ApiMonitor.start() returns an empty (falsy) id, so every downstream mutator
  (append_reply / set_reply / set_usage / finish / fail) short-circuits
  on its existing if not entry_id guard and no entry is ever recorded.
• snapshot(), active_count() and get() return empty / zero / None
  naturally, so the Monitor view stays empty.

It is off by default, so existing behaviour is unchanged and no call sites
needed to change — the singleton is simply constructed as
ApiMonitor(enabled = not _api_monitor_disabled()).

Scope note: "API monitor" here means the in-memory OpenAI-compatible API
activity monitor in core/inference/api_monitor.py, not the HTTP access log
(LoggingMiddleware in loggers/handlers.py). Happy to extend to the access
log too if that was the intent.

Validation

• python -m pytest tests/test_api_monitor.py -> 17 passed (5 new tests:
  disabled-is-noop, enabled-by-default-records, and env-var truthy/falsy/unset
  parsing).
• ruff check studio/backend/core/inference/api_monitor.py studio/backend/tests/test_api_monitor.py -> clean.
• python -m compileall on both touched files -> clean.
• Manual singleton smoke: UNSLOTH_STUDIO_DISABLE_API_MONITOR=1 -> start()
  returns "" and snapshot() is []; unset -> records normally.
• Impact audit: all 9 api_monitor.start() call sites in routes/inference.py
  assign to an Optional[str] monitor_id that is consumed only through the
  guarded mutators, so returning "" when disabled is safe.

Fixes #7036

[gemini-code-assist]

Code Review

This pull request introduces an opt-in kill switch for the in-memory API monitor via the UNSLOTH_STUDIO_DISABLE_API_MONITOR environment variable. When disabled, the ApiMonitor class acts as a no-op, returning a falsy ID on start and preventing any telemetry or logging from being recorded. Comprehensive unit tests have been added to verify the default behavior, the disabled state, and the environment variable parsing. There are no review comments, and I have no additional feedback to provide.

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