feat(worker): allow fixed TP groups in Elastic EP

[thxCode]

What type of PR is this?

/kind enhancement
/area worker

What this PR does / why we need it:

The finalized main implementation allows fixed TP groups in Elastic EP. TP is read from the existing engine extraArgs parser and remains immutable after creation. elasticEp.width changes the DP member count.

• Remove the earlier TP=1 profile restriction without adding a TP API field or a TP=2 upper limit.
• Default and validate each member's whole-GPU quantity against its configured TP size.
• Count only complete TP allocations and Ray placement groups. Reject incomplete, duplicate, ambiguous, or cross-node groups.
• Preserve the master during DP scale-up and reject online changes to the effective TP size. Equivalent argument spellings remain valid.
• Update the existing Elastic EP guide and owning spec. No separate TP spec is retained.

Which issue(s) this PR links to:

NONE

Special notes for your reviewer:

• The original squash merge was superseded on main by e9d9c47c3b8baa1e6289d484daf0f092a0f93b4c through an explicitly authorized history rewrite. This description records the finalized main contract; the original PR head and merge record remain historical.
• Scale-up and unchanged width are supported. Scale-down remains outside this change. Online TP adjustment is not supported.
• CPU regression cases cover TP=1, 2, 4, and 8, argument parsing and immutable updates, complete-card allocation, and Ray placement counterexamples. These cases do not establish physical support for every model or hardware shape.
• The full race/shuffle/coverage suite passed in a clean source export. Core tests, source/docs lint, generation, and independent read-only review also passed.
• The published amd64 image and a live CPU worker binary matched the finalized source commit. The preemptible eight-GPU test environment reported NotEnoughResources; TP=2 inference and DP2-to-DP4 resize did not execute. The test resources were fully cleaned up.
• The related infrastructure recovery documentation is in PR 744.

Does this PR introduce a user-facing change?

Elastic EP members can use fixed TP settings from engine extraArgs. Width changes the DP member count, and each member holds a complete TP GPU group. TP remains fixed after creation.

[gpustack-code-review]

✅ OpenCodeReview: Review complete: 0 finding(s) across 2 selected item(s).

Reviewed 257dea5..5105958 only; earlier commits in this PR were reviewed in a previous run.

[thxCode]

Review round 1 is addressed at 5105958.

Read all feedback buckets: one top-level review, three inline findings, and two additional findings in the original summary. The incremental review now reports zero new findings.

Finding	Outcome
API generation reminder	Generated API artifacts are included. Regeneration leaves the current tree unchanged; verify-generated CI passed.
Empty placement-bundle resources	Kept the fail-closed contract. CPU-only bundles with stated CPU resources pass. Missing resources remain Unknown because they cannot establish GPU bundle cardinality. Missing actor/node identity is refused by the join before this helper.
Webhook generation and tests reminder	Generated webhook output remains unchanged. Admission/default/immutability regression tests are included.
Empty sibling allocation records	Fixed in 50fd8e0. Empty sibling records are accepted. Sibling containers that hold accelerators are still refused with a specific reason. A regression test failed before the fix and passed after it.
Guide validation rationale	Updated in 5105958. The guide keeps the model compatibility requirement and links to the owning specification for validation scope.

Validation after the fixes: clean-source full make test with race/shuffle, make lint, make lint docs, and make generate all passed. Physical TP=2 testing remains pending the merged CI image.