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

[he-yufeng]

I checked the remaining red matrix job. It is checks / test (3.13, lowest-direct, ubuntu-latest), failing in ests/issues/test_1363_race_condition_streamable_http.py::test_race_condition_invalid_accept_headers with RuntimeError: Task group is not initialized. Make sure to use run().\n\nThis PR only narrows MCPServer.call_tool result typing and updates the corresponding focused tests; the failure is in the streamable HTTP race-condition test path and the aggregate �ll-green job is passing. I am treating it as unrelated to this change unless maintainers see a connection I missed.

[he-yufeng]

Rebased this PR onto current upstream/main and force-pushed 468c9c0 to refresh CI after the unrelated Python 3.13 lowest-direct failure.

Validated locally after the rebase:

PYTHONUTF8=1 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
# 10 passed

PYTHONUTF8=1 uv run --frozen pytest tests\interaction\test_coverage.py -q
# 5 passed

PYTHONUTF8=1 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
PYTHONUTF8=1 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
PYTHONUTF8=1 uv run --frozen python -m py_compile 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 upstream/main..HEAD