feat(mcp): add client property into MCPServer

[Kludex]

Note

Adds MCPServer.client property to access the underlying ClientSession when running, with docs and tests verifying safe access.

• MCP Core (pydantic_ai/mcp.py):
  • New API: MCPServer.client property exposes the underlying ClientSession; raises AttributeError if accessed outside the async context.
  • Init refactor: Store ClientSession on self._client immediately and enter it via the exit stack in __aenter__.
• Docs (docs/mcp/client.md):
  • New section: How to access the underlying client with examples for listing/reading prompts/resources and context-manager caveats.
• Tests (tests/test_mcp.py):
  • New test: Validates MCPServerStdio.client access behavior (error before context; usable inside), and reading resources.
  • Minor imports added for snapshots and URL types.

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[Kludex]

When I run:

pytest -k 'test_docs_examples[docs/mcp/client.md:630:manual_client_access.py]'

It seems the server doesn't have any resources/prompts at all. Ideas @DouweM ?

[DouweM]

@Kludex Code looks good.

The docs examples use a mock MCP server:

pydantic-ai/tests/test_examples.py

Lines 283 to 300 in ce6bf90

	class MockMCPServer(AbstractToolset[Any]):
	@property
	def id(self) -> str | None:
	return None # pragma: no cover
	async def __aenter__(self) -> MockMCPServer:
	return self
	async def __aexit__(self, *args: Any) -> None:
	pass
	async def get_tools(self, ctx: RunContext[Any]) -> dict[str, ToolsetTool[Any]]:
	return {}
	async def call_tool(
	self, name: str, tool_args: dict[str, Any], ctx: RunContext[Any], tool: ToolsetTool[Any]
	) -> Any:
	return None # pragma: lax no cover