fix(utils): handle non-ImportError exceptions when checking eventlet contextvars

[agu2347]

Problem

_is_contextvars_broken() in sentry_sdk/utils.py only catches ImportError when checking whether eventlet has monkeypatched the stdlib:

try:
    import greenlet
    from eventlet.patcher import is_monkey_patched
    ...
except ImportError:
    pass

Depending on the exact combination of eventlet, greenlet, and other monkeypatched packages installed (e.g. dnspython, httpcore), importing greenlet/eventlet can raise something other than ImportError (e.g. AttributeError) while eventlet's monkeypatching machinery is initializing. Since _is_contextvars_broken() is called once at import time to set HAS_REAL_CONTEXTVARS, ContextVar = _get_contextvars(), an uncaught exception here crashes the SDK on import for anyone hitting one of these combinations.

Additionally, when an import fails partway through, Python still leaves a (broken) module object registered in sys.modules, so a later import eventlet/import greenlet from application code would silently reuse the broken cached module instead of retrying cleanly.

Closes #7202

Fix

• Broaden the except ImportError around the eventlet/greenlet import to also catch Exception, so an unexpected error there degrades gracefully (treated as "not broken") instead of crashing the SDK.
• On such an exception, remove any modules that got newly added to sys.modules during the failed import attempt, so a subsequent import isn't stuck with a broken partial module.

Testing

Added test_is_contextvars_broken_survives_eventlet_attributeerror, which mocks builtins.__import__ to simulate the exact crash reported in #7202 (an AttributeError raised while importing greenlet, with a partial module left in sys.modules), without needing the actual fragile eventlet+trio+httpcore+dnspython version combination installed.

• Confirmed the new test fails with the reported AttributeError when the fix is reverted (stashed sentry_sdk/utils.py only, test unchanged).
• All 5 tests in tests/utils/test_contextvars.py pass with the fix applied.
• ruff format --check and ruff check pass on both changed files.