fix: support Anthropic-compatible endpoints for third-party providers (#1997)

Three bugs prevented providers like MiniMax from using their
Anthropic-compatible endpoints (e.g. api.minimax.io/anthropic):

1. _VALID_API_MODES was missing 'anthropic_messages', so explicit
   api_mode config was silently rejected and defaulted to
   chat_completions.

2. API-key provider resolution hardcoded api_mode to 'chat_completions'
   without checking model config or detecting Anthropic-compatible URLs.

3. run_agent.py auto-detection only recognized api.anthropic.com, not
   third-party endpoints using the /anthropic URL convention.

Fixes:
- Add 'anthropic_messages' to _VALID_API_MODES
- API-key providers now check model config api_mode and auto-detect
  URLs ending in /anthropic
- run_agent.py and fallback logic detect /anthropic URL convention
- 5 new tests covering all scenarios

Users can now either:
- Set MINIMAX_BASE_URL=https://api.minimax.io/anthropic (auto-detected)
- Set api_mode: anthropic_messages in model config (explicit)
- Use custom_providers with api_mode: anthropic_messages

Co-authored-by: Test <test@test.com>

run_agent.py

@@ -493,6 +493,11 @@ class AIAgent:
         elif self.provider == "anthropic" or (provider_name is None and "api.anthropic.com" in self._base_url_lower):
             self.api_mode = "anthropic_messages"
             self.provider = "anthropic"
+        elif self._base_url_lower.rstrip("/").endswith("/anthropic"):
+            # Third-party Anthropic-compatible endpoints (e.g. MiniMax, DashScope)
+            # use a URL convention ending in /anthropic. Auto-detect these so the
+            # Anthropic Messages API adapter is used instead of chat completions.
+            self.api_mode = "anthropic_messages"
         else:
             self.api_mode = "chat_completions"
 
@@ -3474,11 +3479,11 @@ class AIAgent:
 
             # Determine api_mode from provider
             fb_api_mode = "chat_completions"
+            fb_base_url = str(fb_client.base_url)
             if fb_provider == "openai-codex":
                 fb_api_mode = "codex_responses"
-            elif fb_provider == "anthropic":
+            elif fb_provider == "anthropic" or fb_base_url.rstrip("/").lower().endswith("/anthropic"):
                 fb_api_mode = "anthropic_messages"
-            fb_base_url = str(fb_client.base_url)
 
             old_model = self.model
             self.model = fb_model