feat(initializer): validate initializer Pod uses correct volume name

[HarshPopat23]

What this PR does / why we need it ?:

Adds validation in the JobSet plugin's Validate() function to ensure the
dataset/model initializer ReplicatedJob has a Volume named
jobsetplgconsts.VolumeNameInitializer.

Previously VolumeNameInitializer was only a string constant with no
enforcement — a misconfigured runtime template could silently reference
the wrong volume name and only fail later at Pod scheduling/mount time,
instead of at admission time with a clear error.

This follows the same pattern already used in this function for validating
container names (rJobContainerNames), extended to also track volume
names (rJobVolumeNames) per ReplicatedJob.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #3710

Checklist:

• Docs included if any changes are user facing (not user facing — internal validation logic only, no API/docs change needed)

[Copilot]

Pull request overview

This PR strengthens admission-time validation in the JobSet runtime plugin so misconfigured dataset/model initializer templates fail fast (with a clear validation error) rather than later during Pod scheduling/mount.

Changes:

• Adds tracking/validation of volume names per ReplicatedJob and requires the initializer volume name to be jobsetplgconsts.VolumeNameInitializer.
• Updates JobSet plugin validation tests to assert the new volume-name error cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
pkg/runtime/framework/plugins/jobset/jobset.go	Adds initializer-volume name validation to Validate() for dataset/model initializer jobs.
pkg/runtime/framework/plugins/jobset/jobset_test.go	Extends validation test expectations and adds cases covering missing initializer volume.

[andreyvelich]

/ok-to-test
/retest

[andreyvelich]

Thanks for the contribution, @HarshPopat23! 🙏 A few comments below. Note: this review was performed with the help of AI tools.

[google-oss-prow]

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[HarshPopat23]

Updated the PR to address the Copilot and maintainer review feedback:

• Removed duplicate TODO noise.
• Removed inline Pod Volume validation to avoid rejecting torchtune runtimes that use JobSet volumeClaimPolicies.
• Added initializer container VolumeMount validation using jobsetplgconsts.VolumeNameInitializer.
• Ensured missing initializer ReplicatedJob/container cases do not produce misleading volume/volumeMount errors.
• Added positive tests for volumeClaimPolicies + VolumeMount runtime shape.
• Added a dataset + model initializers together positive test.
• Added negative tests for missing dataset/model initializer VolumeMounts.

Tests:

• gofmt -w pkg/runtime/framework/plugins/jobset/jobset.go pkg/runtime/framework/plugins/jobset/jobset_test.go
• go test ./pkg/runtime/framework/plugins/jobset

I also attempted:

• go test ./pkg/runtime/...

but it failed locally because my Windows disk/cache is full:
"There is not enough space on the disk." The focused JobSet plugin tests pass.