Allow slashes in connection IDs fetched via the Execution API

[justinpakzad]

---

This PR adds a path converter to the get_connection route in the Execution API so that connections whose IDs contain forward slashes can be fetched. Currently, the endpoint returns a 404 when fetching a connection with an ID such as dev/my_conn. This fix is mainly applicable for secrets backends, as connections created via the REST API/CLI do not allow slashes in the conn_id (although the Connection model itself does). An additional test case was added to cover the new behavior.

closes: #57864

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

• Yes (please specify the tool below)

---

• 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.

[SameerMesiah97]

Looks good. Also, this is the second PR I have seen that is addressing this exact same issue (PR #69178 is the first). It may be worth looking at all other API routes that accept user-defined identifiers to see if the same fix can be applied to those too.