fix(server): align call_tool result shapes

[he-yufeng]

Summary

• remove the unreachable raw-dict branch in MCPServer._handle_call_tool
• align MCPServer.call_tool with the result shapes actually returned by the tool manager
• add regression coverage for direct CallToolResult returns and unstructured content-block returns

Fixes #2695

To verify

• uv run --frozen pytest tests\interaction\mcpserver\test_tools.py::test_tool_returning_call_tool_result_passes_through tests\interaction\mcpserver\test_tools.py::test_tool_without_output_schema_returns_unstructured_content tests\interaction\mcpserver\test_tools.py::test_call_tool_model_return_becomes_structured_content tests\server\mcpserver\test_server.py::TestServer::test_call_tool_return_annotation_lists_reachable_shapes -q
• uv run --frozen pytest tests\interaction\test_coverage.py -q
• uv run --frozen ruff check src\mcp\server\mcpserver\server.py tests\interaction\mcpserver\test_tools.py tests\interaction\_requirements.py tests\server\mcpserver\test_server.py
• uv run --frozen pyright src\mcp\server\mcpserver\server.py tests\interaction\mcpserver\test_tools.py tests\interaction\_requirements.py tests\server\mcpserver\test_server.py
• git diff --check