Apply impersonation_chain to deferred BigQuery existence checks

[SEPURI-SAI-KRISHNA]

BigQueryTableExistenceSensor and BigQueryTablePartitionExistenceSensor hand their
impersonation chain to the trigger inside hook_params:

trigger=BigQueryTableExistenceTrigger(
    ...,
    hook_params={"impersonation_chain": self.impersonation_chain},
)

but the trigger authenticates from a different attribute entirely:

def _get_async_hook(self) -> BigQueryTableAsyncHook:
    return BigQueryTableAsyncHook(
        gcp_conn_id=self.gcp_conn_id, impersonation_chain=self.impersonation_chain
    )

Nothing ever sets self.impersonation_chain from those call sites, so it stays None and
the deferred existence check runs as the connection's service account. hook_params is
stored and serialized but never read anywhere in the module — it carries the value and
then drops it.

poke() builds its hook correctly, so the sensor honours impersonation when
deferrable=False and ignores it when deferrable=True. That divergence is the awkward
part: turning deferrable off appears to "fix" the permissions error, which points
investigation away from the real cause.

The trigger already grew a proper impersonation_chain parameter in #36341; the sensors
were simply never moved onto it. This change passes it explicitly at both defer sites.

hook_params is left untouched here. It is a required argument on the trigger's public
__init__, and now that nothing reads it, removing or deprecating it is a separate
decision — happy to follow up if maintainers would like it gone.

For reviewers: GoogleBaseHook.__init__ falls back to the connection's
impersonation_chain extra when the argument is absent, so deployments that configure
impersonation on the connection were never affected. This only bites when impersonation is
set on the sensor, which is the documented parameter.

No newsfragment: this is a provider change, and provider changelogs are regenerated from
git log by the release manager.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 5)

Generated-by: Claude Code (Opus 5) following the guidelines

[SEPURI-SAI-KRISHNA]

Thanks @SameerMesiah97, moved.

The standalone parametrized test is gone. Each sensor now has its own test_deferred_trigger_receives_impersonation_chain sitting next to that class's existing deferral test — in TestBigqueryTableExistenceSensor and in TestBigqueryTablePartitionExistenceSensor.

I dropped the isinstance(exc.value.trigger, ...) assertion during the move: the sibling test_execute_deferred / test_execute_with_deferrable_mode in each class already covers the trigger type, so the new tests assert only the argument that was being lost.

Verified locally: 25 passed with the change, and exactly the 2 new tests fail with it reverted (23 pre-existing tests unaffected either way).

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Drafted-by: Claude Code (Opus 5); reviewed by @SEPURI-SAI-KRISHNA before posting

[SEPURI-SAI-KRISHNA]

Gentle ping, the review feedback here was addressed on 18 August and CI has been green since. Anything else you'd like changed before this can go in?

[potiuk]

Approving. Deferred table and partition existence checks now run with the configured impersonation_chain instead of the triggerer's default credentials; the triggers already supported it but never received it. Thanks!

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Drafted-by: Claude Code (Opus 5.5); reviewed by @potiuk before posting