fix: align MCPServer tool result shapes

[Epochex]

Closes #2695.

Summary

This removes the unreachable raw-dict branch from MCPServer._handle_call_tool() and aligns MCPServer.call_tool() with the shapes that convert_result=True can actually return:

• CallToolResult returned directly by a tool
• a bare Sequence[ContentBlock] for unstructured content
• (Sequence[ContentBlock], dict[str, Any]) when structured output is available

The shared ToolResult alias keeps the public annotation and handler expectations from drifting again.

Validation

uv run pytest tests/server/mcpserver/test_server.py -q
uv run ruff check src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py
uv run ruff format --check src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py
uv run pyright src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py