fix(operator): fix TrainJob suspend/resume webhook error (#3008)

[JEETDESAI25]

What this PR does / why we need it:

• Decouple JobSet suspend toggling from the SSA payload so that the controller no longer trips the JobSet webhook's "spec.replicatedJobs is immutable" error when suspending or resuming existing workloads.
• Add a clarifying comment that suspend for existing JobSets is handled via SyncSuspend, preventing future regressions.

Which issue(s) this PR fixes:

• Fixes TrainJob suspend/resume fails with JobSet webhook validation error #3008

Checklist:

• [ ] Docs included if any changes are user facing

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[github-actions]

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[astefanutti]

/ok-to-test

[coveralls]

Pull Request Test Coverage Report for Build 20355704238

Details

• 24 of 48 (50.0%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 51.469%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/runtime/core/clustertrainingruntime.go	0	3	0.0%
pkg/runtime/core/trainingruntime.go	0	3	0.0%
pkg/controller/trainjob_controller.go	0	8	0.0%
pkg/runtime/framework/plugins/jobset/jobset.go	15	25	60.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/controller/trainjob_controller.go	1	0.0%

Totals
Change from base Build 20289967122:	0.03%
Covered Lines:	1261
Relevant Lines:	2450

---

💛 - Coveralls

[JEETDESAI25]

/assign @terrytangyuan