fix: Execution.create() 503 from credentials annotation typo and missing metadata store init

[javabrett]

Summary

Fixes #6610.

Two bugs in metadata/execution.py caused Execution.create() to fail with 503 Getting metadata from plugin failed with error: before_request:

1. Credentials annotation typo (create() line 103, _create() line 181): Used credentials=Optional[auth_credentials.Credentials] (assignment, =) instead of credentials: Optional[auth_credentials.Credentials] = None (annotation with default, :). The = form makes the default value the typing.Optional type object itself (a _UnionGenericAlias), not None. This non-None object is passed to the gRPC auth stack, which calls .before_request() on it, producing the 503. Present since PR feat: Vertex AI Experiments GA #1410 (June 2022). artifact.py and context.py both have the correct : ... = None syntax.

2. Missing ensure_default_metadata_store_exists() call: Artifact.create() calls metadata_store._MetadataStore.ensure_default_metadata_store_exists() before delegating to _create(), but Execution.create() skipped this. Standalone Execution.create() (without a prior Artifact.create() or aiplatform.init(experiment=...) warming the store) would fail.

Changes

• google/cloud/aiplatform/metadata/execution.py: Fix credentials annotation on both create() and _create(). Add ensure_default_metadata_store_exists() call in create() matching Artifact.create().
• tests/unit/aiplatform/test_metadata_resources.py: 3 regression tests:
  • test_create_credentials_default_is_none -- verifies both create() and _create() default to None
  • test_create_calls_ensure_default_metadata_store_exists -- verifies the store init is called for default store
  • test_create_skips_ensure_for_non_default_metadata_store -- verifies it's skipped for non-default stores

Test plan

• 3 new tests FAIL against unfixed code (2 FAILED, 1 PASSED as expected)
• 3 new tests PASS against fixed code
• All 13 existing TestExecution tests pass (zero regressions)

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