feat(providers): extend request_timeout_seconds to all client paths

Follow-up on top of mvanhorn's cherry-picked commit. Original PR only
wired request_timeout_seconds into the explicit-creds OpenAI branch at
run_agent.py init; router-based implicit auth, native Anthropic, and the
fallback chain were still hardcoded to SDK defaults.

- agent/anthropic_adapter.py: build_anthropic_client() accepts an optional
  timeout kwarg (default 900s preserved when unset/invalid).
- run_agent.py: resolve per-provider/per-model timeout once at init; apply
  to Anthropic native init + post-refresh rebuild + stale/interrupt
  rebuilds + switch_model + _restore_primary_runtime + the OpenAI
  implicit-auth path + _try_activate_fallback (with immediate client
  rebuild so the first fallback request carries the configured timeout).
- tests: cover anthropic adapter kwarg honoring; widen mock signatures
  to accept the new timeout kwarg.
- docs/example: clarify that the knob now applies to every transport,
  the fallback chain, and rebuilds after credential rotation.

tests/run_agent/test_context_token_tracking.py

@@ -40,7 +40,7 @@ class _FakeOpenAIClient:
 def _make_agent(monkeypatch, api_mode, provider, response_fn):
     _patch_bootstrap(monkeypatch)
     if api_mode == "anthropic_messages":
-        monkeypatch.setattr("agent.anthropic_adapter.build_anthropic_client", lambda k, b=None: _FakeAnthropicClient())
+        monkeypatch.setattr("agent.anthropic_adapter.build_anthropic_client", lambda k, b=None, **kwargs: _FakeAnthropicClient())
     if provider == "openai-codex":
         monkeypatch.setattr(
             "agent.auxiliary_client.resolve_provider_client",