fix(shared): make UrlElicitationRequiredError pickle-safe

[ulofiai]

Fixes #2431.

The constructor shape reported for McpError no longer exists on main: MCPError now stores (code, message, data) in Exception.args, so the base class already survives pickle round-trips. UrlElicitationRequiredError still inherits those arguments even though its constructor accepts (elicitations, message=None), causing both pickle and cloudpickle to call it with the wrong arity during unpickling.

Add an explicit __reduce__ that reconstructs the subclass through its original constructor signature. This preserves the exception type, message, structured error payload, and typed elicitation list. Add regression coverage for both the base MCPError contract and the URL-elicitation subclass.

Tests:

• tests/shared/test_exceptions.py (12 passed)
• direct pickle and cloudpickle==3.1.2 round-trips

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}