Add E2E coverage for custom deployment add-revision (FR-3205 command redesign + FR-3342 runtime-variant defaults)

[agatha197]

Add end-to-end (Playwright) test coverage for the custom deployment add-revision flow, covering both:

• FR-3205 (PR feat(FR-3205): redesign custom deployment Start Command with shell/exec modes #8170): redesigned Start Command input — raw-string command/shell field, shell-vs-exec mode UI, optional Start Command, restored Model Definition Path (moved to Advanced).
• FR-3342 (PR feat(FR-3342): drive deployment model-definition placeholders from RuntimeVariant.readsVfolderConfigFiles + defaultModelDefinition #8324): runtime-variant readsVfolderConfigFiles gate migration and defaultModelDefinition two-layer placeholder behavior.

Scope (broad):

• Custom add-revision flow: runtime variant select → model folder → command (shell/exec) → Model Definition Path (Advanced) → submit payload shape.
• Runtime-variant gates: variant selection toggles runtime-parameter presets vs the Service Configuration section; placeholder hints populate from the DB defaultModelDefinition baseline merged under the vfolder model-definition.yaml; the Advanced Model Definition Path value does NOT alter placeholders above it.

Approach: mock-based (setupGraphQLMocks), since FR-3342 fields are gated at manager 26.8.0 and not available on any live backend. Reuse existing e2e deployment mocks/helpers where possible.

Stacking: separate PR stacked on top of PR #8324 (FR-3342), because the FR-3342 behavior can only be exercised with that branch's code present.

JIRA Issue: FR-3344