Return a 503 for specific PDO exceptions, alternate mix

[paul-m]

Fixes [issue#]

Describe your changes

This is another way to do #4704 much more generalized.

• Marks DKAN API routes as DKAN API routes in the yml files.
• Adds an exception event subscriber, so we can handle specific exceptions.
• We're handling the case of a \PDOException with a code of 1203 (too many connections).
• Always return a JsonResponse with a 503 response code for this exception, and a Retry-After header of our choosing.

To see it in action, make a local and modify an API controller method to throw the exception. I suggest Drupal\dkan_datastore\Controller\AbstractQueryController::queryResource. Throw an exception like this:

    throw new \PDOException(
      'AI bots taking over.', 1203
    );

Now request: https://dkan.ddev.site/api/1/datastore/query/8701d18a-5792-5451-8700-a621bccc5f6e

See a message like this:

{
"message": "Try again later after 120 seconds.",
"code": 503
}

Watchdog should show you a message like: "PDOException (code: 1203) AI bots taking over."

QA Steps

• Add manual QA steps in checklist format for a reviewer to perform. Be as specific as possible, provide examples if appropriate.

Checklist before requesting review

If any of these are left unchecked, please provide an explanation

• I have updated or added tests to cover my code
• I have updated or added documentation

[paul-m]

Closed in favor of handling this in the controller: #4704