test: fix three pre-existing failures on Python 3.10 and fastmcp 3.4.3

[Sallvainian]

Three tests fail on master today. The Test Voice Mode MCP workflow is
pull_request-triggered, so master never runs it — the breakage only shows up
on contributors' PRs, where it reads as "your branch broke CI".

Reproduced on a clean master checkout with no other changes present:

$ git checkout 5c1b3e4
$ uv run --python 3.10 -m pytest \
    tests/test_mcp_shutdown_patch.py::TestDispatchUntilClosed::test_cancels_on_iteration_error \
    tests/test_turns_discoverability.py

FAILED ...test_cancels_on_iteration_error - NameError: name 'ExceptionGroup' is not defined
FAILED ...test_turns_schema_property_carries_the_description - KeyError: 'description'
FAILED ...test_turns_schema_description_covers_key_contract_points - KeyError: 'description'
3 failed, 4 passed

ExceptionGroup on Python 3.10

ExceptionGroup only became a builtin in 3.11, so on 3.10 the name is undefined
and the assertion raises NameError instead of testing anything. exceptiongroup
is already locked as a dependency for python_full_version < '3.11' (uv.lock),
so this imports the backport there rather than adding anything new.

turns schema description on fastmcp 3.4.3

The tests read tool.parameters["properties"]["turns"]["description"]. fastmcp
3.4.3 emits an optional parameter as an anyOf union and carries the
description on a nested member, so the key is absent at the top level:

"turns": {"anyOf": [{"anyOf": [...], "description": "Ordered list of utterances..."}], "default": null}

The description is still present and still byte-for-byte correct — only its
position moved. Rather than hardcode the new shape (which just swaps one version
assumption for another), the lookup now walks anyOf members recursively, so it
passes on both the old and new layouts.

test_turns_param_description_constant_matches_readme_literal_text is untouched
and still enforces the golden README text, so the drift guard this file exists
for is unaffected.

Verification

	before	after
these two files, py3.10	3 failed, 4 passed	16 passed
these two files, py3.12	16 passed	16 passed
full suite, py3.10	3 failed	1918 passed, 56 skipped, 0 failed

Test-only; no runtime code touched, so no CHANGELOG entry.

Split out from #517 so the CI fix can land independently of that PR.