fix: add default value validation for AI policy configuration

[zhengkezhou1]

Description

Fixes #11559

Change Type

/kind bug_fix

Changelog

NONE

Additional Notes

[Copilot]

Pull Request Overview

This PR enhances default value handling in AI policy configurations by validating JSON literals and refining template generation.

• Introduces JSON validation for FieldDefault.Value when it appears to be an object or array
• Updates applyDefaults to correctly build Inja templates for override and non-override cases
• Adds comprehensive unit tests for default value processing in ai_policy_test.go

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
internal/kgateway/extensions2/plugins/trafficpolicy/ai_policy.go	Add JSON validity checks and adjust template logic for defaults
internal/kgateway/extensions2/plugins/trafficpolicy/ai_policy_test.go	Add TestDefault cases covering valid/invalid JSON defaults

[zhengkezhou1]

Is there any way to verify the exception when translation fails?

[npolshakova]

You can potentially do a check similar to this if there's an error in the status:

kgateway/internal/kgateway/translator/gateway/gateway_translator_test.go

Line 902 in 7fd6a25

Expect(translatortest.AreReportsSuccess(gwNN, reportsMap)).NotTo(HaveOccurred())

[npolshakova]

I think since the traffic policy doesn't have the error in the status, this can just confirm the output is as expected and the error assertions can happen in ai_policy_test.go

[timflannagan]

@npolshakova I think we'd see an error surface on the TP & HTTPRoute status (now that #11535 has landed), right? preProcessAITrafficPolicy is called during the builder's Translate method and we propagate those errors to the policy IR.

[timflannagan]

Yeah confirmed locally the TP rejects the invalid config from the original issue:

...
conditions:
- lastTransitionTime: "2025-07-09T20:29:22Z"
  message: 'field custom_list contains invalid JSON string: [1,2,3'
  observedGeneration: 1
  reason: Invalid
  status: "False"
  type: Accepted

I think we'd need to fix the merge conflict & merge with main to see the HTTPRoute updated as well. Right now it's reporting an Accepted=true state on my local cluster.

[zhengkezhou1]

@shashankram @npolshakova I've added test cases for non-JSON scenarios and included comments for the new code logic.

[timflannagan]

@zhengkezhou1 Looks like there's some merge conflicts with the gateway translator suite. Can we also merge with main? I think we'd see the HTTPRoute status get affected by this change in addition to the error reflecting on the TP status.