feat: validate Kptfile Inventory and Upstream

[vishnukothakapu]

Description

Description of the change
This PR introduces explicit validation for the Inventory and Upstream fields within the Kptfile API.

• It updates the Kptfile.Validate() method to execute sub-validators for these fields if they are populated.
• For Inventory, it ensures that Name, Namespace, and InventoryID are present using the existing IsValid() method.
• For Upstream, it ensures that a valid Type (git or generic) and a valid UpdateStrategy are specified, and strictly enforces that a repository is provided if the origin is git.
• Includes full unit test coverage for these new validation paths in api/kptfile/v1/validation_test.go.

Motivation for the change
Currently, kpt only validates the Pipeline field of the Kptfile during early parsing, leaving other crucial fields unchecked (as noted by a TODO in the codebase). Hand-editing a Kptfile with malformed Upstream or Inventory data can lead to unexpected and confusing runtime errors much later in the command execution cycle.

By shifting this validation to the earliest possible point (the core Validate() method), we significantly improve the developer experience by catching malformed configurations immediately.

Fixes #4735

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
87.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud