[Multi_K8s-Plugin] Fix livestate loadManifests to stamp tracking labels and honour kustomizeDir

[mohammedfirdouss]

What this PR does:
Fixes loadManifests in livestate/plugin.go to match what deployment/plugin.go already does:

1. Honours kustomizeDir from the per-target multiTarget config previously the field was ignored, so kustomize overlay directories set per-target had no effect in livestate.
2. Stamps the standard tracking labels and annotations (LabelManagedBy, LabelPiped, LabelApplication, LabelCommitHash, LabelOriginalAPIVersion, LabelResourceKey) on every loaded manifest previously these were missing, causing a divergence between the manifests the deployment plugin applies and the manifests livestate uses for drift detection.

Why we need it:
The deployment and livestate plugins use separate loadManifests implementations that had diverged. Missing labels in the livestate version meant drift detection compared against manifests that lacked the builtin PipeCD tracking labels, potentially causing false drift reports or missed drift. Missing kustomizeDir meant per-target kustomize overlays were silently ignored during livestate checks.

Which issue(s) this PR fixes:

Fixes #6446

Does this PR introduce a user-facing change?:

• How are users affected by this change: Livestate drift detection becomes more accurate, it now compares against manifests with the same labels and overlays that were actually applied.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 36.41%. Comparing base (215ea40) to head (11b5bd8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6783      +/-   ##
==========================================
+ Coverage   29.33%   36.41%   +7.07%     
==========================================
  Files         598       97     -501     
  Lines       63902     7981   -55921     
==========================================
- Hits        18744     2906   -15838     
+ Misses      43711     4916   -38795     
+ Partials     1447      159    -1288     

Flag	Coverage Δ
.	?
.-pkg-app-pipedv1-plugin-analysis	32.43% <ø> (ø)
.-pkg-app-pipedv1-plugin-ecs	31.82% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	?
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	?
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	37.95% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	33.04% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	52.71% <ø> (ø)
.-pkg-plugin-sdk	50.34% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	?
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[Warashi]

LGTM

[github-actions]

Thank you for contributing to PipeCD! The changes in this pull request will be part of the upcoming release!