fix(client): release owned dispatcher exception hook

[qdivan]

Summary

• release the hook that ClientSession installs on an injected JSONRPCDispatcher during cleanup
• preserve caller-owned or subsequently replaced hooks via identity-based ownership tracking
• cover exit cleanup, dispatcher reuse, and caller-hook preservation

Tests

• uv run pytest tests/client -q (714 passed, 1 xfailed)
• uv run ruff check src/mcp/client/session.py tests/client/test_session.py
• uv run pyright src/mcp/client/session.py tests/client/test_session.py

Closes no issue: identified by the existing TODO at the dispatcher/session ownership seam.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[github-actions]

Thanks for the contribution. This repository only keeps pull requests open when they're linked to an issue that a maintainer has assigned to the author — CONTRIBUTING.md explains why and how we work. This PR has been closed for now because its description doesn't yet link an open issue in this repository (with Fixes #123 or similar).

If there isn't an issue for this yet, please open one — a clear description of the problem is genuinely the most useful thing for us. Then add Fixes #<number> to this PR's description. If a maintainer would like the change as a PR from you, they'll assign you to the issue and this PR will reopen automatically.

There's no need to open a new PR — this one will be reopened. While it's closed, please push any updates as new commits rather than force-pushing, since GitHub can't reopen a PR whose branch has been rewritten.

Maintainers: reopening this PR, removing the missing-issue-link label, or adding bypass-issue-check bypasses the check.