fix(ndb): use the single-argument generator.throw() signature

[skippdot]

Fixes #18158

Summary

_TaskletFuture._advance_tasklet throws exceptions into the wrapped generator using the three-argument form of generator.throw(), deprecated in Python 3.12:

if error:
    traceback = error.__traceback__
    yielded = self.generator.throw(type(error), error, traceback)

Every exception that crosses a tasklet boundary emits a DeprecationWarning. This library's own unit suite raises 70 warnings today; with this change it raises 36, so 34 of them came from this single line.

Today that is only noise, but the Python docs say the old signature "may be removed in a future version". noxfile.py already lists 3.15 in ALL_INTERPRETERS, so it is worth landing before that removal makes every tasklet-boundary exception raise TypeError instead of propagating.

Changes

The single-argument form reads the traceback off the exception itself, so the local becomes redundant:

if error:
    yielded = self.generator.throw(error)

Also adds a regression test asserting both halves of the contract: the exception still arrives with its __traceback__ intact, and no DeprecationWarning is emitted. I verified the test fails against the old line and passes against the new one, so it genuinely guards the behaviour rather than just tracking it.

Verification

• pytest tests/unit — 1833 passed, 1 skipped (1832 before, plus the new test); warnings drop from 70 to 36
• ruff format --check and flake8 google tests — clean
• Reproduced the original warning with only google-cloud-ndb and the standard library; see google-cloud-ndb: DeprecationWarning from the three-arg generator.throw() in tasklets #18158 for the standalone snippet

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[gemini-code-assist]

Code Review

This pull request simplifies exception propagation in tasklets by replacing the legacy three-argument generator.throw() call with the single-argument version generator.throw(error). This avoids DeprecationWarning in newer Python versions while preserving the traceback. A corresponding regression test has been added to verify this behavior and ensure no deprecation warnings are raised. There are no review comments, and no further feedback is required.

[parthea]

Thanks @skippdot!