fix(anthropic): deep scan fixes — auth, retries, edge cases

Fixes from comprehensive code review and cross-referencing with
clawdbot/OpenCode implementations:

CRITICAL:
- Add one-shot guard (anthropic_auth_retry_attempted) to prevent
  infinite 401 retry loops when credentials keep changing
- Fix _is_oauth_token(): managed keys from ~/.claude.json are NOT
  regular API keys (don't start with sk-ant-api). Inverted the logic:
  only sk-ant-api* is treated as API key auth, everything else uses
  Bearer auth + oauth beta headers

HIGH:
- Wrap json.loads(args) in try/except in message conversion — malformed
  tool_call arguments no longer crash the entire conversation
- Raise AuthError in runtime_provider when no Anthropic token found
  (was silently passing empty string, causing confusing API errors)
- Remove broken _try_anthropic() from auxiliary vision chain — the
  centralized router creates an OpenAI client for api_key providers
  which doesn't work with Anthropic's Messages API

MEDIUM:
- Handle empty assistant message content — Anthropic rejects empty
  content blocks, now inserts '(empty)' placeholder
- Fix setup.py existing_key logic — set to 'KEEP' sentinel instead
  of None to prevent falling through to the auth choice prompt
- Add debug logging to _fetch_anthropic_models on failure

Tests: 43 adapter tests (2 new for token detection), 3197 total passed

tests/test_anthropic_adapter.py

@@ -33,6 +33,14 @@ class TestIsOAuthToken:
     def test_api_key(self):
         assert _is_oauth_token("sk-ant-api03-abcdef1234567890") is False
 
+    def test_managed_key(self):
+        # Managed keys from ~/.claude.json are NOT regular API keys
+        assert _is_oauth_token("ou1R1z-ft0A-bDeZ9wAA") is True
+
+    def test_jwt_token(self):
+        # JWTs from OAuth flow
+        assert _is_oauth_token("eyJhbGciOiJSUzI1NiJ9.test") is True
+
     def test_empty(self):
         assert _is_oauth_token("") is False