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

[anishgirianish]

---

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

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.