fix(api): Prevent AssertionError from oversized DSN project_id in error-page embed

[sentry]

This PR addresses SENTRY-5H8J, where an `AssertionError` was raised in `BoundedBigAutoField.get_prep_value` when processing DSNs with an extremely large `project_id`.

The ProjectKey.from_dsn method, used by the /api/embed/error-page/ endpoint, was extracting the project_id from the DSN path and passing it directly to ProjectKey.objects.get(). While ValueError for non-integer IDs was handled, a valid integer exceeding BoundedBigAutoField.MAX_VALUE would trigger an AssertionError during the ORM's preparation of the value.

To fix this, ProjectKey.from_dsn now explicitly validates the extracted project_id:

1. It attempts to convert the project_id to an integer.
2. It checks if the integer is positive and within the bounds of BoundedBigAutoField.MAX_VALUE.
3. If the project_id is invalid (non-integer, non-positive, or out-of-range), it raises ProjectKey.DoesNotExist.

This ensures that the AssertionError is prevented, and invalid DSNs are gracefully handled by the calling endpoint, which already maps ProjectKey.DoesNotExist to a 404 response.

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Fixes SENTRY-5H8J