[Multi_K8s-Plugin] Pin Is* methods to exact Kubernetes API groups

[mohammedfirdouss]

What this PR does:
Tightens the API group checks in provider/manifest.go. The Is* methods previously used isBuiltinAPIGroup() which accepted any of ~20 builtin Kubernetes API groups, a Deployment-like CRD from a non-apps group would pass IsDeployment(). Each method is now pinned to its exact API group:

• IsDeployment, IsStatefulSet, IsDaemonSet, IsReplicaSet → group must be "apps"
• IsService, IsSecret, IsConfigMap, IsPod → group must be "" (core)
• IsWorkload → applies the per-Kind group rule above

isBuiltinAPIGroup and the builtinAPIGroups map are now unused and have been removed.

Why we need it:
The loose check could misidentify a custom resource with the same Kind string but a different API group as a known Kubernetes workload or service, causing incorrect variant label injection or cleanup behaviour.

Which issue(s) this PR fixes:

Fixes #6446

Does this PR introduce a user-facing change?:

• How are users affected by this change: No change for standard Kubernetes resources. Users with CRDs that share a Kind name with a builtin resource (e.g. a CRD named Deployment in a non-apps group) will no longer have that resource treated as a workload which is the correct behaviour.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.65%. Comparing base (215ea40) to head (3dec18e).

Files with missing lines	Patch %	Lines
...lugin/kubernetes_multicluster/provider/manifest.go	36.36%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6784       +/-   ##
===========================================
+ Coverage   29.33%   56.65%   +27.32%     
===========================================
  Files         598       99      -499     
  Lines       63902    10496    -53406     
===========================================
- Hits        18744     5947    -12797     
+ Misses      43711     4115    -39596     
+ Partials     1447      434     -1013     

Flag	Coverage Δ
.	?
.-pkg-app-pipedv1-plugin-analysis	?
.-pkg-app-pipedv1-plugin-ecs	?
.-pkg-app-pipedv1-plugin-kubernetes	58.37% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	61.66% <36.36%> (-0.24%)	⬇️
.-pkg-app-pipedv1-plugin-scriptrun	?
.-pkg-app-pipedv1-plugin-terraform	37.95% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	?
.-pkg-app-pipedv1-plugin-waitapproval	?
.-pkg-plugin-sdk	50.14% <ø> (-0.20%)	⬇️
.-tool-actions-gh-release	?
.-tool-actions-plan-preview	?
.-tool-codegen-protoc-gen-auth	?

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[Warashi]

Sorry for late, LGTM

[github-actions]

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