fix(Dashboard): Sync/Async Dashboard Screenshot Generation

[geido]

SUMMARY

PR #29187 introduced a new endpoint for generating dashboard screenshots and PR #29272 made this endpoint mandatory for the frontend. However, Celery should be an optional dependency in Superset and the endpoint is dependent on Celery to work. This PR makes Celery optional for generating dashboard screenshots by:

• Reintroducing the previous download screenshot functionality:
  This update enables dashboards to be downloaded directly from the browser, without using the dedicated API endpoint. This approach has known CORS limitations but will work for most cases where Celery isn't configured.

• Adding two new feature flags:

  • ENABLE_DASHBOARD_SCREENSHOT_ENDPOINTS:
    Controls whether the API endpoint that caches and retrieves dashboard screenshots via a webdriver is enabled, allowing this functionality to be decoupled from the UI. This flag is set to False by default.

  • ENABLE_DASHBOARD_DOWNLOAD_WEBDRIVER_SCREENSHOT:
    Controls whether the UI’s download functionality uses webdriver-based screenshot generation. This flag is also False by default and it is dependent on ENABLE_DASHBOARD_SCREENSHOT_ENDPOINTS.

Fixes #30645

TESTING INSTRUCTIONS

1. Enter a Dashboard
2. Download a PDF or Image screenshot via the Dashboard menu
3. The screenshot should download normally

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags: ENABLE_DASHBOARD_SCREENSHOT_ENDPOINTS and ENABLE_DASHBOARD_DOWNLOAD_WEBDRIVER_SCREENSHOT
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[geido]

Moving this to draft as we might change the approach here, bring back the previous frontend-generated screenshots while having the feature flag to enable Selenium generated screenshots optionally.

[michael-s-molina]

Thanks for the updated version of the PR @geido. Do you think we need ENABLE_DASHBOARD_SCREENSHOT_ENDPOINTS? I'm ok with the endpoint being exposed all the time. We have the same characteristic for other endpoints where the feature might not be enabled client-side. Maybe only ENABLE_DASHBOARD_DOWNLOAD_WEBDRIVER_SCREENSHOT is sufficient?

[geido]

Thanks for the updated version of the PR @geido. Do you think we need ENABLE_DASHBOARD_SCREENSHOT_ENDPOINTS? I'm ok with the endpoint being exposed all the time. We have the same characteristic for other endpoints where the feature might not be enabled client-side. Maybe only ENABLE_DASHBOARD_DOWNLOAD_WEBDRIVER_SCREENSHOT is sufficient?

This is to be consistent with the thumbnails endpoint that are also behind feature flag. In general, I think it is good to be intentional about enabling these endpoints as they expose performance-heavy functionality.

[geido]

/testenv up

[github-actions]

@geido Ephemeral environment spinning up at http://35.90.38.171:8080. Credentials are admin/admin. Please allow several minutes for bootstrapping and startup.

[michael-s-molina]

Thank you so much for the fix @geido!

[Vitor-Avila]

LGTM!

[villebro]

Can we have a link to the PR that caused this regression? Just to make it easier to discover and understand how we can avoid these types of things in the future..

[michael-s-molina]

Ok, I think the exception for the rgbcolor license resolved CI. @villebro we need an owner approval 🙏🏼

[rusackas]

Approving license override

[geido]

Can we have a link to the PR that caused this regression? Just to make it easier to discover and understand how we can avoid these types of things in the future..

Description updated

[github-actions]

Ephemeral environment shutdown and build artifacts deleted.