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.

agent/anthropic_adapter.py

@@ -292,9 +292,15 @@ def _common_betas_for_base_url(base_url: str | None) -> list[str]:
     return _COMMON_BETAS
 
 
-def build_anthropic_client(api_key: str, base_url: str = None):
+def build_anthropic_client(api_key: str, base_url: str = None, timeout: float = None):
     """Create an Anthropic client, auto-detecting setup-tokens vs API keys.
 
+    If *timeout* is provided it overrides the default 900s read timeout.  The
+    connect timeout stays at 10s.  Callers pass this from the per-provider /
+    per-model ``request_timeout_seconds`` config so Anthropic-native and
+    Anthropic-compatible providers respect the same knob as OpenAI-wire
+    providers.
+
     Returns an anthropic.Anthropic instance.
     """
     if _anthropic_sdk is None:
@@ -305,8 +311,9 @@ def build_anthropic_client(api_key: str, base_url: str = None):
     from httpx import Timeout
 
     normalized_base_url = _normalize_base_url_text(base_url)
+    _read_timeout = timeout if (isinstance(timeout, (int, float)) and timeout > 0) else 900.0
     kwargs = {
-        "timeout": Timeout(timeout=900.0, connect=10.0),
+        "timeout": Timeout(timeout=float(_read_timeout), connect=10.0),
     }
     if normalized_base_url:
         kwargs["base_url"] = normalized_base_url