Make SQLAlchemy an optional dependency for JDBC and YDB providers

[FrankYang0529]

Followup for #59895.

for those providers where sqlalchemy is just for some feature, we should make sure that they handle lack of sqlalchemy installed - handing import error, and raising "Optional Provider Feature" exception when a method that needs sqlalchemy is callsed and sqlalchemy is not installed. Sqlalchemy should be added as "extra" sqlalchemy dependency.

Move the module-level from sqlalchemy.engine import URL into if TYPE_CHECKING: and only import it inside sqlalchemy_url.

---

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

• Yes - Claude Code

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.