fix(shared): make UrlElicitationRequiredError pickle-safe (refs #2431)

[MukundaKatta]

References #2431.

Bug

The default Exception unpickle path reconstructs via cls(*self.args). MCPError.__init__ sets args to (code, message, data) — three positionals. That doesn't match UrlElicitationRequiredError's signature (elicitations, message=None), so unpickling raises:

TypeError: UrlElicitationRequiredError.__init__() takes from 2 to 3 positional arguments but 4 were given

Reproduces on stdlib pickle and cloudpickle alike. Repro on current main:

import pickle
from mcp.shared.exceptions import UrlElicitationRequiredError
from mcp.types import ElicitRequestURLParams

err = UrlElicitationRequiredError([
    ElicitRequestURLParams(mode="url", message="auth", url="https://e.com", elicitation_id="x"),
])
pickle.loads(pickle.dumps(err))  # ↑ TypeError

Fix

Add __reduce__ that reconstructs from the typed (elicitations, message) pair, so the round-trip preserves the high-level field rather than the wire-format data dict.

Tests

• New test_url_elicitation_required_error_pickle_round_trip — exercises a 2-elicitation error, asserts class identity, message, elicitations list, and reconstructed error.code.
• New test_mcp_error_pickle_round_trip — pins the parent class's already-working round trip so a future __init__ refactor can't silently break it.
• 11/11 passing in tests/shared/test_exceptions.py.

Note on issue scope

#2431 describes a related but slightly different constructor shape (McpError(error: ErrorData)) that no longer exists on main — the parent class was refactored to (code, message, data=None) since the report. That refactor accidentally fixed the parent's pickle round-trip but left the URL-elicitation subclass broken on a different code path. This PR closes that remaining gap.