Fixing mypy errors in google gen ai provider and tests

[amoghrajesh]

---

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

• Yes (please specify the tool below)

Several mypy errors across three providers were failing CI type checks, see in https://github.com/apache/airflow/actions/runs/26211631196/job/77125309480?pr=67118

providers/google/tests/unit/google/cloud/hooks/test_gen_ai.py:79: error: Argument "contents" to "CreateCachedContentConfig" has incompatible type "list[Content]"; expected
  "Content | str | Image | File | Part | list[str | Image | File | Part] | list[Content | str | Image | File | Part | list[str | Image | File | Part]] | None"  [arg-type]
          contents=[
                   ^
  providers/google/tests/unit/google/cloud/hooks/test_gen_ai.py:79: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
  providers/google/tests/unit/google/cloud/hooks/test_gen_ai.py:79: note: Consider using "Sequence" instead, which is covariant
  providers/amazon/src/airflow/providers/amazon/aws/executors/batch/batch_executor.py:434: error: Argument "key" to "BatchQueuedJob" has incompatible type "TaskInstanceKey | str"; expected
  "TaskInstanceKey | CallbackKey"  [arg-type]
                      key=key,
                          ^~~
  providers/google/tests/unit/google/cloud/operators/test_gen_ai.py:64: error: Argument "contents" to "CreateCachedContentConfig" has incompatible type "list[Content]"; expected
  "Content | str | Image | File | Part | list[str | Image | File | Part] | list[Content | str | Image | File | Part | list[str | Image | File | Part]] | None"  [arg-type]
          contents=[
                   ^
  providers/google/tests/unit/google/cloud/operators/test_gen_ai.py:64: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
  providers/google/tests/unit/google/cloud/operators/test_gen_ai.py:64: note: Consider using "Sequence" instead, which is covariant
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:203: error: Argument "contents" to "CreateCachedContentConfig" has incompatible type "list[Content]"; expected
  "Content | str | Image | File | Part | list[str | Image | File | Part] | list[Content | str | Image | File | Part | list[str | Image | File | Part]] | None"  [arg-type]
          contents=[
                   ^
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:203: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:203: note: Consider using "Sequence" instead, which is covariant
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:259: error: Argument "contents" to "GenAICountTokensOperator" has incompatible type "list[str]"; expected
  "Content | str | Image | File | Part | list[str | Image | File | Part] | list[Content | str | Image | File | Part | list[str | Image | File | Part]] | Content | ContentDict | str | Image | File | FileDict | Part | PartDict | list[str | Image | File | FileDict | Part | PartDict] | list[Content | ContentDict | str | Image | File | FileDict | Part | PartDict | list[str | Image | File | FileDict | Part | PartDict]]"
   [arg-type]
              contents=CONTENTS,
                       ^~~~~~~~
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:269: error: Argument "contents" to "GenAIGenerateContentOperator" has incompatible type "list[str]"; expected
  "Content | ContentDict | str | Image | File | FileDict | Part | PartDict | list[str | Image | File | FileDict | Part | PartDict] | list[Content | ContentDict | str | Image | File | FileDict | Part | PartDict | list[str | Image | File | FileDict | Part | PartDict]]"
   [arg-type]
              contents=CONTENTS,
                       ^~~~~~~~
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:269: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:269: note: Consider using "Sequence" instead, which is covariant
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:327: error: Argument "contents" to "GenAIGenerateContentOperator" has incompatible type "list[str]"; expected
  "Content | ContentDict | str | Image | File | FileDict | Part | PartDict | list[str | Image | File | FileDict | Part | PartDict] | list[Content | ContentDict | str | Image | File | FileDict | Part | PartDict | list[str | Image | File | FileDict | Part | PartDict]]"
   [arg-type]
              contents=["What are the papers about?"],
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:327: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
  providers/google/tests/system/google/cloud/gen_ai/example_gen_ai_generative_model.py:327: note: Consider using "Sequence" instead, which is covariant

Fix is to:

• add | list[str] to the contents parameter of GenAIGenerateContentOperator and GenAICountTokensOperator, aligning them with GenAIGenerateEmbeddingsOperator.
• Three gen_ai test files: extract the [Content(...)] literal into a _CACHED_CONTENTS: Sequence[Content] typed variable before passing it to CreateCachedContentConfig, fixing the list invariance error per mypy's own recommendation.

The batch executor execute_async key type mismatch (TaskInstanceKey | str vs TaskInstanceKey | CallbackKey) is tracked separately in PR #67269.

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[amoghrajesh]

Failing mypy is unrelated and Anish has a fix for it in #67268