fix: make URL elicitation errors pickle-safe

[mikemikimike]

Summary

UrlElicitationRequiredError could not be unpickled because the inherited exception state stores (code, message, data), while its constructor expects an elicitation list and optional message.

This change adds a __reduce__ implementation that reconstructs the exception through the existing from_error method, preserving the wire error payload and elicitation details.

Validation

• uv run --frozen pytest tests/shared/test_exceptions.py (11 passed)
• uv run --frozen ruff check src/mcp/shared/exceptions.py tests/shared/test_exceptions.py
• uv run --frozen pyright src/mcp/shared/exceptions.py tests/shared/test_exceptions.py
• git diff --check

The full test suite was also run; it has one unrelated Windows environment failure in tests/client/test_transport_stream_cleanup.py::test_sse_client_closes_all_streams_on_connection_error, where the local proxy returned HTTP 502 instead of the expected connection error.

Fixes #2431.

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #2431.

If a maintainer assigns you to #2431, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.