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

agent/auxiliary_client.py

@@ -449,21 +449,6 @@ def _try_custom_endpoint() -> Tuple[Optional[OpenAI], Optional[str]]:
     return OpenAI(api_key=custom_key, base_url=custom_base), model
 
 
-_ANTHROPIC_VISION_MODEL = "claude-sonnet-4-20250514"
-
-
-def _try_anthropic() -> Tuple[Optional[Any], Optional[str]]:
-    """Try Anthropic credentials for auxiliary tasks (vision-capable)."""
-    from agent.anthropic_adapter import resolve_anthropic_token
-    token = resolve_anthropic_token()
-    if not token:
-        return None, None
-    # Return a simple wrapper that indicates Anthropic is available.
-    # The actual client is created by resolve_provider_client("anthropic").
-    logger.debug("Auxiliary client: Anthropic (%s)", _ANTHROPIC_VISION_MODEL)
-    return resolve_provider_client("anthropic", model=_ANTHROPIC_VISION_MODEL)
-
-
 def _try_codex() -> Tuple[Optional[Any], Optional[str]]:
     codex_token = _read_codex_access_token()
     if not codex_token:
@@ -768,8 +753,8 @@ def get_vision_auxiliary_client() -> Tuple[Optional[OpenAI], Optional[str]]:
     # back to the user's custom endpoint.  Many local models (Qwen-VL,
     # LLaVA, Pixtral, etc.) support vision — skipping them entirely
     # caused silent failures for local-only users.
-    for try_fn in (_try_openrouter, _try_nous, _try_anthropic,
-                   _try_codex, _try_custom_endpoint):
+    for try_fn in (_try_openrouter, _try_nous, _try_codex,
+                   _try_custom_endpoint):
         client, model = try_fn()
         if client is not None:
             return client, model