## What does this PR do?
Adds an end-to-end test for the Feature Opt-In banner flow. This test verifies the complete user journey from seeing the banner to enabling a feature.
Currently `OPT_IN_FEATURES` in `packages/features/feature-opt-in/config.ts` is empty, so this e2e test skips. Once we uncomment the `bookings-v3` item from the config, this e2e test will run and pass. This is a preparation for when we enable the feature.
**Changes:**
- Creates a new e2e test file (`feature-opt-in-banner.e2e.ts`) that tests the complete opt-in flow
- Adds `data-testid` attributes to banner and dialog components for reliable test selectors
## Mandatory Tasks (DO NOT REMOVE)
- [x] I have self-reviewed the code (A decent size PR without self-review might be rejected).
- [x] I have updated the developer docs in /docs if this PR makes changes that would require a [documentation change](https://cal.com/docs). N/A - test only.
- [x] I confirm automated tests are in place that prove my fix is effective or that my feature works.
## How should this be tested?
1. Add the `ready-for-e2e` label to run e2e tests in CI
2. The test should pass when run with `PLAYWRIGHT_HEADLESS=1 yarn e2e feature-opt-in-banner.e2e.ts`
**Test requirements:**
- The `bookings-v3` feature flag must exist in the Features table (the test uses `prisma.feature.upsert` to ensure this)
- The test will skip if `bookings-v3` is not in `OPT_IN_FEATURES`
## Checklist
- [x] I have read the [contributing guide](https://github.com/calcom/cal.com/blob/main/CONTRIBUTING.md)
- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have checked if my changes generate no new warnings
## Updates since last revision
- Replaced text-based locators (`getByText`, `getByRole`) with `data-testid` selectors for more reliable element targeting
- Added `data-testid` attributes to `FeatureOptInBanner`, `FeatureOptInConfirmDialog`, and `FeatureOptInSuccessDialog` components
## Human Review Checklist
- [ ] Verify the `data-testid` attributes don't conflict with existing test IDs in the codebase
- [ ] Confirm the test correctly skips when `bookings-v3` is not in `OPT_IN_FEATURES`
- [ ] Check that the added `data-testid` attributes follow the project's naming conventions
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Link to Devin run: https://app.devin.ai/sessions/97158ab1b7414e3988ba803b30d95b3e
Requested by: @eunjae-lee
