Remove dead dict branch in _handle_call_tool and fix call_tool return type

`FuncMetadata.convert_result` only ever returns one of three shapes:
a `CallToolResult`, a `Sequence[ContentBlock]`, or a
`(unstructured_content, structured_content)` tuple. It never returns a
raw `dict`, so the `isinstance(result, dict)` branch in
`_handle_call_tool` was unreachable (already marked `# pragma: no cover`
with a TODO).

Remove the dead branch, correct `call_tool`'s return annotation to
reflect the three real shapes, and drop the now-orphaned `json` import
(used only inside the removed branch).

Closes #2695

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: agaonker

src/mcp/server/mcpserver/server.py

@@ -4,7 +4,6 @@
 
 import base64
 import inspect
-import json
 import re
 from collections.abc import AsyncIterator, Awaitable, Callable, Iterable, Sequence
 from contextlib import AbstractAsyncContextManager, asynccontextmanager
@@ -322,14 +321,6 @@ async def _handle_call_tool(
                 content=list(unstructured_content),  # type: ignore[arg-type]
                 structured_content=structured_content,  # type: ignore[arg-type]
             )
-        if isinstance(result, dict):  # pragma: no cover
-            # TODO: this code path is unreachable — convert_result never returns a raw dict.
-            # The call_tool return type (Sequence[ContentBlock] | dict[str, Any]) is wrong
-            # and needs to be cleaned up.
-            return CallToolResult(
-                content=[TextContent(type="text", text=json.dumps(result, indent=2))],
-                structured_content=result,
-            )
         return CallToolResult(content=list(result))
 
     async def _handle_list_resources(
@@ -399,7 +390,7 @@ async def list_tools(self) -> list[MCPTool]:
 
     async def call_tool(
         self, name: str, arguments: dict[str, Any], context: Context[LifespanResultT, Any] | None = None
-    ) -> Sequence[ContentBlock] | dict[str, Any]:
+    ) -> CallToolResult | Sequence[ContentBlock] | tuple[Sequence[ContentBlock], dict[str, Any]]:
         """Call a tool by name with arguments."""
         if context is None:
             context = Context(mcp_server=self)