Add validate option

[mmarchetti]

Intent

This PR adds a validation step after deployment, as in posit-dev/rsconnect-python#511.

Validation is the default but can be disabled by setting validate = false in the deployment configuration.

Fixes #437

Type of Change

• Bug Fix
• New Feature
• Breaking Change

Approach

The Connect client has a new ValidateDeployment method that accesses the deployed content using the user's credentials. This will fail if the app cannot successfully start (for example, it has an import error, or raises an exception in code that runs during import).

This also adds a new event phase publish/validateDeployment for the validation step. Since validation takes a couple seconds, this should have its own phase in the progress UI.

Automated Tests

Updated existing tests to cover this new step.

Directions for Reviewers

Deploy an app and see the validation step in the log:

time=2023-12-04T17:45:42.087-05:00 level=INFO msg="Validating deployment" event_op=publish/validateDeployment event_phase=start
time=2023-12-04T17:45:44.315-05:00 level=INFO msg=Done event_op=publish/validateDeployment event_phase=success ok=true

Deploy an app that imports a nonexistent package (so it fails startup), and see a validation error:

time=2023-12-04T17:42:25.406-05:00 level=INFO msg="Validating deployment" event_op=publish/validateDeployment event_phase=start
time=2023-12-04T17:42:27.337-05:00 level=ERROR msg="unexpected response from the server" event_op=publish/validateDeployment event_phase=failure error_code=serverErr URL=https://rsc.radixu.com/content/43485fe8-b037-4f2a-9f7e-e604d5347a1c Method=GET Status=500

Set validate = false in default.toml, and you should not see any validation activity in the logs.

[dotNomad]

Created #456 as a follow-up to this.