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.

cli-config.yaml.example

@@ -67,6 +67,10 @@ model:
 
 # Named provider overrides (optional)
 # Use this for per-provider request timeouts and per-model exceptions.
+# Applies to the primary turn client on every api_mode (OpenAI-wire, native
+# Anthropic, and Anthropic-compatible providers), the fallback chain, and
+# client rebuilds during credential rotation.  Leaving these unset keeps the
+# SDK defaults (OpenAI ≈ 600s, native Anthropic 900s).
 #
 # providers:
 #   ollama-local: