stdio sessions hang forever when a server never responds (no default read_timeout_seconds)

## What happened? `_create_stdio_session` (`langchain_mcp_adapters/sessions.py`) sets no default `read_timeout_seconds` on the `ClientSession` it constructs, and neither the MCP SDK's stdio transport nor this library set one anywhere else in the stdio path. A server that spawns successfully but never writes a JSON-RPC response leaves `initialize()`/`call_tool()`/`list_tools()` awaiting forever, with no way for the caller to recover short of an external kill. **AI disclosure:** this investigation was prepared with AI assistance (Claude Code) as part of a broader architectural review of this library's MCP trust boundary (see langchain-ai/langchain#40473 for the related hardening RFC). I've reproduced it myself and can walk through any part of it. ## Reproduction A minimal "server" that reads one line from stdin (so it doesn't fail with a broken pipe) and then just sleeps: ```python # hanging_server.py import sys, time sys.stdin.readline() time.sleep(3600) ``` ```python import anyio, sys from langchain_mcp_adapters.sessions import _create_stdio_session async def main(): async with _create_stdio_session(command=sys.executable, args=["hanging_server.py"]) as session: with anyio.fail_after(3): await session.initialize() # hangs past this bound with no read_timeout_seconds anyio.run(main) ``` This raises `TimeoutError` from the *test's own* `anyio.fail_after(3)` — nothing internal to `_create_stdio_session`/`ClientSession` bounds the wait on its own. ## Expected behavior A per-request timeout should be set by default (the MCP SDK's `ClientSession` already accepts `read_timeout_seconds: timedelta | None` for exactly this purpose, via `BaseSession.send_request`'s own `anyio.fail_after`), so a non-responsive server surfaces as a catchable `McpError` instead of hanging the caller indefinitely. ## Proposed fix (ready pending approval) Default `read_timeout_seconds` to 60s in `_create_stdio_session`, merged via `dict.setdefault()` so any caller-supplied …