Use built-in exceptions for Postgres hook input validation

[mitre88]

Part of the ongoing clean-up of broad AirflowException usages (enforced by the check-no-new-airflow-exceptions ratchet), following the pattern of #66279.

PostgresHook raised AirflowException for two input-validation failures: a sqlalchemy_query connection extra that is not a dict (now TypeError), and requesting realdictcursor with psycopg3 (now ValueError, matching the invalid-cursor branch immediately below it that already raises ValueError). The known_airflow_exceptions.txt entry for the file drops from 2 to 0, and the existing test now asserts the specific exception type and message.

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

• Yes — Claude Code (Fable 5)

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

[potiuk]

Thanks — and the type selection here is better than most of this migration.

Splitting the two raises by kind rather than defaulting both to ValueError is the right call: sqlalchemy_query not being a dict is a type error, while realdictcursor is a valid-type-but-unsupported value. That distinction usually gets flattened. The realdictcursor change also now matches its immediate neighbour, which already raises ValueError(f"Invalid cursor passed {_cursor}...") — so the two adjacent failure paths finally agree.

Removing the ratchet entry outright is right too: it was ::2 and both occurrences are gone, so the line goes rather than being decremented to a stale count. The match= on the updated test is a genuine strengthening.

One small gap: the realdictcursor branch has no test asserting the new type. The only realdict reference in test_postgres.py is a monkeypatch.setattr(..., "RealDictCursor", None) at line 69 — nothing exercises the raise. So half this PR is covered and half isn't. It's a message-preserving type change so the risk is low, and a two-line pytest.raises(ValueError, match="realdictcursor is not supported") around hook._get_cursor("realdictcursor") would close it whenever you next touch the file. Not blocking.

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Drafted-by: Claude Code (Opus 5); reviewed by @potiuk before posting