fix: normalize httpx.URL base_url + strip thinking signatures for third-party endpoints

Two linked fixes for MiniMax Anthropic-compatible fallback:

1. Normalize httpx.URL to str before calling .rstrip() in auth/provider
   detection helpers. Some client objects expose base_url as httpx.URL,
   not str — crashed with AttributeError in _requires_bearer_auth() and
   _is_third_party_anthropic_endpoint(). Also fixes _try_activate_fallback()
   to use the already-stringified fb_base_url instead of raw httpx.URL.

2. Strip Anthropic-proprietary thinking block signatures when targeting
   third-party Anthropic-compatible endpoints (MiniMax, Azure AI Foundry,
   self-hosted proxies). These endpoints cannot validate Anthropic's
   signatures and reject them with HTTP 400 'Invalid signature in
   thinking block'. Now threads base_url through convert_messages_to_anthropic()
   → build_anthropic_kwargs() so signature management is endpoint-aware.

Based on PR #4945 by kshitijk4poor (rstrip fix).
Fixes #4944.

run_agent.py

@@ -4895,7 +4895,7 @@ class AIAgent:
                 effective_key = (fb_client.api_key or resolve_anthropic_token() or "") if fb_provider == "anthropic" else (fb_client.api_key or "")
                 self.api_key = effective_key
                 self._anthropic_api_key = effective_key
-                self._anthropic_base_url = getattr(fb_client, "base_url", None)
+                self._anthropic_base_url = fb_base_url
                 self._anthropic_client = build_anthropic_client(effective_key, self._anthropic_base_url)
                 self._is_anthropic_oauth = _is_oauth_token(effective_key)
                 self.client = None
@@ -5334,6 +5334,7 @@ class AIAgent:
                 is_oauth=self._is_anthropic_oauth,
                 preserve_dots=self._anthropic_preserve_dots(),
                 context_length=ctx_len,
+                base_url=getattr(self, "_anthropic_base_url", None),
             )
 
         if self.api_mode == "codex_responses":