Rate-limit Bugzilla requests per run in bugzilla-proxy #7331

bugzilla-proxy requests are exempted from BMO's rate limiting. With that protection gone, a runaway or looping agent could send BMO unbounded traffic under bugzilla-proxy credentials.

The proxy should enforce its own per-run budget instead:

• Key the budget on the token's sub (the run id), so one run cannot starve the others.
• Over budget, return a Bugzilla-shaped 429 saying when to retry, so agents can back off.
• Record throttled requests in the audit log.

Applies to both the passthrough (unprivileged) and scoped (privileged) modes.