Document literal-bool limitation of tools/call error detection

Expand the comment to note raw-dict isError is matched as a literal bool;
non-bool coercible values that serialize to an error are left undetected,
since no hand predicate matches pydantic's wire coercion.

Author: Kludex

src/mcp/server/_otel.py

@@ -59,7 +59,10 @@ async def __call__(self, ctx: ServerRequestContext[Any, Any], call_next: CallNex
                 span.set_status(StatusCode.ERROR, str(e))
                 raise
             if ctx.method == "tools/call":
-                # Only shapes that survive wire serialization (alias-only) are real tool errors.
+                # Tool errors are detected pre-serialization, so only shapes that reach the wire as an error
+                # count: the model, or the camelCase alias (`is_error` is dropped by the alias-only wire
+                # validation). A raw-dict `isError` is matched as a literal bool only - non-bool coercible
+                # values (1, "true") would serialize to an error but are rare enough to leave undetected.
                 match result:
                     case CallToolResult(is_error=True) | {"isError": True}:
                         span.set_attribute("error.type", "tool_error")