test(playground): mark playground-shareable-url as @stable

[daniellicnerski1]

Summary

• Migrates playground-shareable-url.spec.ts setup from the Simple Agent template to setupPlayground(), removing the dependency on a starter template name (the gating contract is hasIO, satisfied by Chat Input → Chat Output).
• Adds @stable to the test tags and updates the spec doc's External dependencies to describe the real hasIO / ENABLE_PUBLISH gating instead of the previous template-tied wording.
• Marks the Shareable Playground row in QA-CHECKLIST.md as covered ([-] → [x]).

Closes #147

Validation pipeline (7 steps)

1. npm run typecheck — clean
2. npx eslint on the spec file — clean
3. playwright-test-linter skill — checklist all green (imports, no false-positive patterns, expect/test ratio 6:1, getByTestId, tags, serial mode)
4. npx playwright test ... --retries=0 — 1 passed (9.9s)
5. Force-fail (regex changed to INTENTIONALLY_BROKEN_FORCE_FAIL) — failed with the actual /playground/{uuid} href shown in the error, then reverted
6. --trace=on — trace generated; first step "create blank Chat Input → Chat Output flow" + Share dropdown interactions match the spec doc
7. No 🚨 Backend Error printed during the run

Test plan

• Run the spec locally with --retries=0 against langflowai/langflow-nightly:latest
• Verify the test fails meaningfully when the URL regex is invalidated
• Confirm trace step labels match the spec doc's validation criteria
• Confirm no 4xx/5xx backend errors are emitted by the fixture

[Copilot]

Pull request overview

This PR improves the long-term stability of the Shareable Playground URL e2e test by removing its dependency on a specific starter template name and aligning documentation/checklists with the actual gating behavior.

Changes:

• Migrated playground-shareable-url.spec.ts setup to use the shared setupPlayground() helper (Chat Input → Chat Output) instead of selecting the “Simple Agent” template.
• Added the @stable tag to the Shareable Playground URL test and updated the corresponding spec documentation to reflect real gating (hasIO / ENABLE_PUBLISH).
• Marked the Shareable Playground checklist item as covered in QA-CHECKLIST.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/tests-automations/regression/core-functionality/playground/playground-shareable-url.spec.ts	Switches test setup to setupPlayground() and adds @stable tag.
QA-CHECKLIST.md	Marks Shareable Playground URL generation as covered.
docs/core-functionality/playground/playground-shareable-url.md	Updates tags and documents the hasIO/ENABLE_PUBLISH gating and relevant test IDs.

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