Test Saved Connection

Live-test a saved connection with its stored credentials.

Any connection the caller may use can be tested, including one shared with them; anything else answers 404. An integration with no live check answers not_testable. When the validation service can't give a verdict, the answer is 503. The result is returned, not stored.

Each test signs in with the owner's credentials, so each caller can test a connection a few times a minute, and the users it is shared with a few times a minute together; more answers 429, sparing the owner's account provider-side lockouts. Shared users can't use up the owner's tests. Only a test that will sign in counts: one that is not testable, has a malformed value or an internal address doesn't. The counts are kept per backend process, like the repository's other request limits.