pgvector: Guard the psycopg2-specific register_vector import

[Dev-iL]

related:

• Remove psycopg2 and migrate the sync Postgres driver to psycopg3 #68453
• pgvector operator's register_vector always assumes a psycopg2 connection #69443

depends on:

• Postgres: Make psycopg (v3) the default synchronous Postgres driver #69474

from pgvector.psycopg2 import register_vector ran unconditionally at module level in PgVectorIngestOperator, forcing psycopg2 to be importable regardless of which driver the underlying PostgresHook connection actually uses. Make the import lazy and raise a clear, actionable error if it's genuinely needed but missing.

Note: this module only imports cleanly without psycopg2 installed once apache-airflow-providers-postgres itself stops hard-requiring psycopg2 — that's the companion change in psycopg3-sync-postgres. This PR should be merged after (or at least released alongside) that one; see the code comment at the import site and in the test file.

This does not fix a separate, pre-existing bug where register_vector is always called with the psycopg2-specific helper regardless of whether the connection is actually psycopg2 or psycopg3 — that's tracked separately at #69443, referenced in a code comment at the guard site so it isn't lost.

What changed

• providers/pgvector/src/airflow/providers/pgvector/operators/pgvector.py: from pgvector.psycopg2 import register_vector moved inside _register_vector(), guarded with AirflowOptionalProviderFeatureException if pgvector.psycopg2/psycopg2 is unavailable.

Test plan

• New tests: test_register_vector_raises_clear_error_without_psycopg2, test_pgvector_module_imports_without_psycopg2 (simulates psycopg2 absent via sys.modules patching, confirms clean import and a clear error on actual use).
• Existing register_vector-mocking tests still pass; a code comment documents that they currently rely on psycopg2 staying installed in this provider's dev environment until the postgres provider's [psycopg2] extra exists to depend on explicitly.

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