[API] /connections/test: Use stored password for saved connections and fix DBT hook URL

[anishgirianish]

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This PR fixes the /connections/test route in Airflow v3 to handle masked passwords correctly and ensures DBT hooks test connections without errors.

What/Why

• Use stored password for saved connections instead of the masked "***".
• If the connection is not saved, create a transient in-memory connection from the request body.
• Fix DBT hook to use the correct connection URL during testing and avoid 400 errors.

How

• Fetch the connection from the DB if connection_id is provided; otherwise, create a transient connection.
• Added unit tests to cover saved and transient connections.
• Updated dbt hook to use correct url

Impact

• Fixes saved and transient connection testing failures and resolves DBT hook 400 errors.
• Earlier versions are also affected and will require separate backports or PRs. The issue is specifically reported in 2.x version

closes: #58941
related: #58941

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[pierrejeambrun]

@anishgirianish Why did you close this PR, do you plan to work on a followup? I think this still needs to be fixed.

[pierrejeambrun]

Ah maybe because there was #59643 opened already. Cool

[anishgirianish]

Hi @pierrejeambrun Apologies for replying late. thank you so much for your reply. I figured only after opening this pr that there was already a Pr open with similar approach as this. Also going through discussion I saw desirable way of implementing it and thought closing this out. I am very intrested to work on a proposal for the discussed approach in #59643 if work has not been started on that. If so would like to request you for the assignment thank you.

[pierrejeambrun]

@anishgirianish You don't need any assignment. You can just start working on a PR, I'd be glad to review it for the short term fix so people have a workaround while we work on the long term fix option