fix(mcp): restrict server prefix to ASCII and bound prefixed names to 64 chars

- `_server_tool_name_prefix`: add `char.isascii()` guard so non-ASCII
  Unicode alphanumerics (e.g. CJK, Arabic) are replaced with `_` instead
  of being passed through — OpenAI function names must be ASCII only.
- `_prefixed_tool_name`: cap the generated name at 64 characters; when
  the full name exceeds the limit a deterministic 8-char SHA-1 suffix is
  appended after truncation to keep names collision-resistant.

Addresses review feedback on #2788.

Author: IgnazioDS

src/agents/mcp/util.py

@@ -277,7 +277,8 @@ async def get_function_tools(
     @staticmethod
     def _server_tool_name_prefix(server_name: str) -> str:
         normalized = "".join(
-            char if char.isalnum() or char in ("_", "-") else "_" for char in server_name
+            char if (char.isascii() and char.isalnum()) or char in ("_", "-") else "_"
+            for char in server_name
         )
         normalized = normalized.strip("_-")
         if not normalized:
@@ -286,7 +287,14 @@ def _server_tool_name_prefix(server_name: str) -> str:
 
     @staticmethod
     def _prefixed_tool_name(tool_name_prefix: str, tool_name: str) -> str:
-        return f"mcp_{len(tool_name_prefix)}_{tool_name_prefix}{tool_name}"
+        full_name = f"mcp_{len(tool_name_prefix)}_{tool_name_prefix}{tool_name}"
+        if len(full_name) <= 64:
+            return full_name
+        # Truncate to 64 chars using a deterministic hash suffix to avoid collisions
+        hash_suffix = hashlib.sha1(full_name.encode("utf-8")).hexdigest()[:8]
+        # Reserve 9 chars for "_" + 8-char hash
+        truncated = full_name[: 64 - 9]
+        return f"{truncated}_{hash_suffix}"
 
     @classmethod
     def _server_tool_name_prefixes(cls, servers: list[MCPServer]) -> dict[int, str]: