Allow keeping PostgresHook SQLAlchemy engines on psycopg2

[sehajsandhu123]

Since postgres provider 7.0.0, PostgresHook builds its SQLAlchemy engines on psycopg (v3) whenever SQLAlchemy 2.x is installed, and the 7.0.0 changelog notes there is no connection or configuration option to keep hooks on psycopg2.

This silently changes semantics for Dag code that uses hook.get_uri() or hook.get_sqlalchemy_engine(). The psycopg (v3) dialect renders typed bind casts, so string parameters that PostgreSQL used to coerce implicitly now fail server-side. The most common casualty is pandas.DataFrame.to_sql into a table with a uuid column, which worked for years on psycopg2 and now fails with:

psycopg.errors.DatatypeMismatch: column "id" is of type uuid but expression is of type character varying
LINE 1: INSERT INTO orders (id, item) VALUES ($1::VARCHAR, $2...

This PR adds the missing opt-out by honoring the sqlalchemy_scheme connection extra (and an equivalent hook parameter), the same convention OdbcHook and DbApiHook.dialect_name already follow. PostgresHook even lists sqlalchemy_scheme in ignored_extra_options, so it is already excluded from libpq connect args — it just wasn't used when building the URL. With this change, a connection that needs the old behaviour can set:

{"sqlalchemy_scheme": "postgresql+psycopg2"}

The value is validated to be postgresql or postgresql+<driver> with no : or /, so a connection extra can't smuggle in a different URL. Nothing changes for connections that don't set it.

Tested against PostgreSQL 17 with pandas 3.0.5 / SQLAlchemy 2.0.51: df.to_sql into a uuid column reproduces the error above on the default psycopg3 engine and succeeds with the extra set. Unit tests cover the override, parameter precedence, get_uri propagation, and rejection of invalid schemes.

related: #71977

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

• Yes — Claude Code (Fable 5); all changes reviewed and validated by the author

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

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