fix(agent): preemptively pace low RPM buckets

[konsisumer]

What changed and why

Per the follow-up on #7489, this is a clean, smaller successor to Refs #7490. It captures rate-limit headers for non-streaming OpenAI-compatible calls, parses OpenAI compact durations and Anthropic RFC 3339 reset values, then delays the next request when a matching request bucket has at most two requests remaining.

The captured state is in-memory and is accepted only when both the active provider and normalized base URL still match, so a provider or route switch cannot inherit an unrelated bucket. OpenRouter is intentionally not enabled by default because successful routed responses do not reliably provide the necessary headers. A custom provider may opt in with rpm_throttle_threshold on its configured route.

Scope justification: the 5-file / 347-line diff is limited to the existing rate-limit state model (parsing plus a pure, interruptible wait), the two existing API entry paths (non-streaming capture and preflight), the one capture call that records route identity, custom-provider normalization/lookup, and focused regressions. It adds no new manager, provider integration, dependency, or configuration migration.

How to test

• pytest -q --timeout=60 tests/agent/test_rate_limit_tracker.py tests/agent/test_cron_inline_api_call_62151.py tests/run_agent/test_anthropic_response_header_capture.py (16 passed)
• ruff check agent/rate_limit_tracker.py agent/chat_completion_helpers.py run_agent.py hermes_cli/config.py tests/agent/test_rate_limit_tracker.py
• python -m py_compile agent/rate_limit_tracker.py agent/chat_completion_helpers.py run_agent.py hermes_cli/config.py tests/agent/test_rate_limit_tracker.py

The requested full suite was attempted, but collection is blocked in this worktree because FastAPI is absent and its automatic install is rejected by the externally managed Homebrew Python.

What platforms tested on

• macOS (Homebrew Python 3.14.6)

Related reference

• feat(agent): pre-emptive RPM throttling using x-ratelimit response headers #7490 is closed and was used only to identify the narrower successor requirements.

Refs #7489

[konsisumer]

Rebased onto current origin/main and resolved the shared streaming entrypoint by preserving upstream stream-lifecycle helpers alongside the RPM preflight. The PR remains its original five-file scope.

Focused regressions: 16 passed. Ruff, Windows-footgun scan, bytecode compilation, and git diff --check pass. The full suite cannot collect in this sandbox because FastAPI is absent and its automatic install is blocked.