feat(mcpserver): expose public get_tool(name) on MCPServer

[arimu1]

Summary

Adds a public MCPServer.get_tool(name) -> Tool | None that delegates to the existing ToolManager.get_tool, so callers can look up a registered tool without using the private _tool_manager.

This completes the public tool registration surface next to add_tool() / remove_tool().

Use case (from #3162): after registration, update a tool's inputSchema (e.g. attach oneOf / discriminated action schemas) via the returned tool's mutable parameters dict.

tool = mcp.get_tool("act")
if tool is not None:
    tool.parameters = {...}  # advertised on tools/list

Fixes #3162

Notes on issue discussion

Maintainer asked whether customizing inputSchema at registration time would be enough. This PR still implements the issue's proposed API (get_tool) because:

1. It is the minimal CRUD completion (add / get / remove)
2. It enables post-registration inspection and mutation without new registration kwargs
3. An optional input_schema= override on add_tool / @tool remains a possible follow-up and is orthogonal

Test plan

• test_get_tool_returns_registered_tool_or_none
• test_get_tool_exposes_mutable_parameters_for_schema_updates
• Neighboring TestServerTools add/remove tests still pass
• ruff + pyright clean on touched files

uv run --frozen pytest \
  tests/server/mcpserver/test_server.py::test_get_tool_returns_registered_tool_or_none \
  tests/server/mcpserver/test_server.py::test_get_tool_exposes_mutable_parameters_for_schema_updates \
  -q

AI disclosure

Assisted by AI (Grok / Cursor) for implementation and tests; human-reviewed before submit.

[arimu1]

Recreating as non-draft (API could not convert draft→ready under current token permissions).