Show clear error for invalid sql_alchemy_conn at startup

[GayathriSrividya]

closes: #68707

Use this PR description:

Title:
Show clear error for invalid sql_alchemy_conn at startup

Body:

Summary

This PR improves startup error handling when database.sql_alchemy_conn is invalid.

Changes included:

1. Added validation in settings URI conversion so malformed connection URIs fail with a clear, actionable ValueError.
2. Error message now points users to both AIRFLOW__DATABASE__SQL_ALCHEMY_CONN and sql_alchemy_conn in airflow.cfg.
3. Refactored URI-conversion tests into parametrized cases for supported schemes and invalid inputs.
4. Kept PostgreSQL test expectations compatible with the active async driver selection (psycopg_async vs asyncpg).

Why

Previously, malformed SQLAlchemy connection URIs could fail in a less clear way during startup. This makes failures easier to diagnose and faster to fix.

Testing

1. Updated unit tests in airflow-core tests for settings URI conversion.
2. Ruff format/check run on modified files.
3. Local targeted pytest via uv was blocked by a provider import issue in this environment; changes are covered by updated unit tests and will be validated in CI.

Reviewer

Requested reviewer: @justinpakzad