fix(client): Await the in-flight stdio session after a cancelled connect waiter

[mikamikasuki]

Description

Retain the in-flight ClientSession future on StdioTransport so a later connect_session() caller can await startup after the original waiter is cancelled. Clear it on disconnect and preserve the existing ready-session connect() behavior.

Closes #

Contribution type

• Bug fix (simple, well-scoped fix for a clearly broken behavior)
• Documentation improvement
• Enhancement

Checklist

• This PR addresses an existing issue (or fixes a self-evident bug)
• I have read CONTRIBUTING.md
• I have added tests that cover my changes
• I have run uv run prek run --all-files and all checks pass
• I have self-reviewed my changes
• If I used an LLM, it followed the repo's contributing conventions (not generic output)

[github-actions]

This pull request was closed because it needs a valid issue link. Please update this PR rather than opening a new one.

Per CONTRIBUTING.md, an external PR must reference an issue that's assigned to its author. To get there:

1. Find or open an issue describing the change — if you open it, you have first claim on it.
2. Add Fixes #<issue>, Closes #<issue>, or Resolves #<issue> to this PR's description — edit it in place, don't open a new PR.

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Maintainers: reopen this PR or remove the missing-issue-link label to bypass this check.