[v1.x] fix: make MCP errors pickle-safe

[jstar0]

Summary

• Add explicit pickle reconstruction for McpError using its complete ErrorData payload.
• Add specialized reconstruction for UrlElicitationRequiredError through its existing from_error parser.
• Add regression tests covering payload, message, and URL elicitation preservation across standard-library pickle round-trips.

Fixes #2431

Verification

• uv run --frozen pytest tests/shared/test_exceptions.py tests/shared/test_session.py tests/client/test_stdio.py -q
• NO_PROXY="*" no_proxy="*" ALL_PROXY="" all_proxy="" HTTP_PROXY="" http_proxy="" HTTPS_PROXY="" https_proxy="" ./scripts/test (1184 passed, 95 skipped, 1 xfailed, 100.00% coverage)
• uv run --frozen pyright
• uv run --frozen ruff check src/mcp/shared/exceptions.py tests/shared/test_exceptions.py

AI assistance was used during investigation and drafting; I reviewed the implementation, regression coverage, and verification results personally.

[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.