Allow slashes in connection IDs fetched via the Execution API

[justinpakzad]

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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

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