fix: preserve array nulls in drift patches

[puneetdixit200]

Refers to #5020

Preserve arrays while normalizing null-only map fields in drift patches. JSON merge patch arrays are replacement values, so recursively deleting nulls inside them can hide real drift such as container imagePullPolicy changes. Map-level null cleanup still handles Helm/default noise.

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Additional Information

Tested:

• go test ./internal/cmd/agent/deployer/desiredset -run 'Test_Diff_NullPatch|Test_Diff_RemoveNullPatchFields' -count=1\n- go test ./internal/cmd/agent/deployer/desiredset -run 'Test_Diff_HPA' -count=1\n- go test ./internal/cmd/agent/deployer/desiredset -count=1\n- go test -shuffle=on ./internal/cmd/agent/deployer/desiredset -count=1\n- git diff --check\n\n### Checklist\n\n- [ ] I have updated the documentation via a pull request in the fleet-product-docs repository. Not applicable to this bug fix.

[Copilot]

Pull request overview

Fixes a drift-detection regression (#5020) by adjusting null-normalization for JSON merge patch diffs so that arrays are preserved rather than recursively “cleaned,” avoiding cases where null-stripping inside arrays can hide real drift in keyed lists (e.g., container specs).

Changes:

• Update removeNullPatchFields to preserve arrays as-is while still removing null-valued fields from map/object nodes.
• Update and extend null-normalization tests to assert that nulls inside arrays (including null elements and null object fields) are preserved.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
internal/cmd/agent/deployer/desiredset/diff.go	Changes null-normalization logic to stop recursing into arrays to avoid masking drift in list-like fields.
internal/cmd/agent/deployer/desiredset/diff_null_test.go	Updates expectations and adds coverage to ensure nulls inside arrays are preserved.

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[puneetdixit200]

AI-assisted follow-up on behalf of Puneet: pushed 2f8807d to clarify the removeNullPatchFields doc comment so it matches the array-preserving behavior.

Verified:

• go test ./internal/cmd/agent/deployer/desiredset -run 'Test_Diff_NullPatch|Test_Diff_RemoveNullPatchFields' -count=1\n- gofmt -w internal/cmd/agent/deployer/desiredset/diff.go\n- git diff --check