Major refactor: API, providers, messaging, and Anthropic protocol

Consolidates the incremental refactor work into a single change set: modular web tools (api/web_tools), native Anthropic request building and SSE block policy, OpenAI conversion and error handling, provider transports and rate limiting, messaging handler and tree queue, safe logging, smoke tests, and broad test coverage.

providers/error_mapping.py

@@ -14,10 +14,30 @@ from providers.exceptions import (
 from providers.rate_limit import GlobalRateLimiter
 
 
+def user_visible_message_for_mapped_provider_error(
+    mapped: Exception,
+    *,
+    provider_name: str,
+    read_timeout_s: float | None,
+) -> str:
+    """Return the user-visible string after :func:`map_error` (405 + mapped types)."""
+    if getattr(mapped, "status_code", None) == 405:
+        return (
+            f"Upstream provider {provider_name} rejected the request method "
+            "or endpoint (HTTP 405)."
+        )
+    return get_user_facing_error_message(mapped, read_timeout_s=read_timeout_s)
+
+
 def map_error(
     e: Exception, *, rate_limiter: GlobalRateLimiter | None = None
 ) -> Exception:
-    """Map OpenAI or HTTPX exception to specific ProviderError."""
+    """Map OpenAI or HTTPX exception to specific ProviderError.
+
+    Streaming transports should pass their scoped limiter (``self._global_rate_limiter``)
+    so reactive 429 handling applies to the correct provider. Tests may omit
+    ``rate_limiter`` to use the process-wide singleton.
+    """
     message = get_user_facing_error_message(e)
     limiter = rate_limiter or GlobalRateLimiter.get_instance()