fix: make MCPError subclasses pickle-safe

[mturac]

Summary

UrlElicitationRequiredError and NoBackChannelError failed to unpickle
because their inherited Exception.args from MCPError are
(code, message, data), which does not match their own __init__
signatures. This caused TypeError on restore and lost subclass-specific
state.

The fix adds __reduce__ to each subclass:

• UrlElicitationRequiredError reconstructs via the existing from_error
  classmethod.
• NoBackChannelError reconstructs via its (method) constructor.

Pickle round-trip tests are added for MCPError,
UrlElicitationRequiredError, and NoBackChannelError.

Fixes #2431

---

This PR was prepared with the assistance of AI coding tools. The analysis,
patch, and tests were reviewed by a human contributor before submission.
Happy to iterate on feedback.

[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.