fix(shared): make McpError and UrlElicitationRequiredError pickle-safe on v1.x

[NishchayMahor]

Description

`McpError.init(error)` calls `super().init(error.message)`, which sets `Exception.args` to `(message_str,)`. The default pickle path reconstructs an exception by calling `cls(*self.args)`, so unpickling a `McpError` invokes `McpError(message_str)` and crashes with:

```text
AttributeError: 'str' object has no attribute 'message'
```

`UrlElicitationRequiredError` inherits the same `args` and additionally has a different signature `(elicitations, message=None)`, so its unpickle path also binds the message string to the `elicitations` parameter and errors out the same way.

This PR adds `reduce` to both classes so pickle reconstructs from the typed `ErrorData` / `_elicitations` fields, and the round-trip preserves the high-level payload rather than the raw wire-format `args`. Existing call sites and the `from_error` constructor are unchanged.

Repro on `v1.x`

```python
import pickle
from mcp.shared.exceptions import McpError, UrlElicitationRequiredError
from mcp.types import ElicitRequestURLParams, ErrorData

McpError — fails on unpickle

pickle.loads(pickle.dumps(McpError(ErrorData(code=-32600, message="Invalid Request"))))

AttributeError: 'str' object has no attribute 'message'

UrlElicitationRequiredError — fails on unpickle

e = UrlElicitationRequiredError([ElicitRequestURLParams(
mode="url", message="x", url="https://example.com", elicitationId="t-1"
)])
pickle.loads(pickle.dumps(e))

TypeError: takes from 2 to 3 positional arguments but 4 were given

```

Scope vs. existing work

A partial fix already landed on `main` for `McpError` (pickle-safe there now). PR #2555 is in review for `UrlElicitationRequiredError` on `main`. This change covers the `v1.x` release branch where both classes are still broken — per the Anthropic-team comment on #2431 ("both `McpError` and `UrlElicitationRequiredError` fail on v1.x").

If maintainers prefer to land this as a forward-port from main's `McpError` patch plus #2555's approach for `UrlElicitationRequiredError`, I can rebase or close — happy either way.

Issue

Github-Issue: #2431

Tests

Added 4 regression tests in `tests/shared/test_exceptions.py`:

• `TestUrlElicitationRequiredError::test_pickle_roundtrip_preserves_elicitations_and_message` — single elicitation + custom message survives the round-trip with typed `ElicitRequestURLParams`.
• `TestUrlElicitationRequiredError::test_pickle_roundtrip_preserves_multiple_elicitations` — multi-elicitation list stays intact and remains typed (not raw dicts).
• `TestMcpErrorPickle::test_pickle_roundtrip_preserves_error_data` — full `ErrorData` (code + message + data) round-trips.
• `TestMcpErrorPickle::test_pickle_roundtrip_without_data_field` — `ErrorData` with no `data` field round-trips cleanly.

All four fail on `v1.x` without this patch and pass with it. The existing 9 tests in the file stay green.

```
$ uv run pytest tests/shared/test_exceptions.py
========= 13 passed in 0.16s =========

$ uv run ruff check src/mcp/shared/exceptions.py tests/shared/test_exceptions.py
All checks passed!

$ uv run pyright src/mcp/shared/exceptions.py tests/shared/test_exceptions.py
0 errors, 0 warnings, 0 informations
```