feat(bubble): migrate saved bubble charts to bubble_v2 and drop the legacy plugin

[rusackas]

SUMMARY

Tier 3 of #41714 (targets remove-legacy-viz-pipeline): retire the nvd3 Bubble Chart (bubble) by migrating saved charts to the ECharts Bubble Chart (bubble_v2).

• The MigrateBubbleChart processor (with CLI registration and unit tests) has existed since the ECharts bubble shipped, but no Alembic revision ever invoked it — so legacy bubble charts kept rendering through explore_json. This PR adds the revision: d4e5f6a7b8c9, with the standard MySQL MEDIUMTEXT widening and reversible form_data_bak backups (same shape as the treemap/sunburst/5.0-batch migrations).
• Removes the nvd3 Bubble plugin (legacy-preset-chart-nvd3/src/Bubble/), its MainPreset + preset registrations, the dead BubbleViz backend class, and the bubble branch of the shared NVD3 renderer.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Saved legacy bubble charts re-open as ECharts Bubble Charts (the same auto-migration path already used for line/area/bar/dist_bar in 5.0).

TESTING INSTRUCTIONS

• pytest tests/unit_tests/migrations/viz/nvd3_bubble_chart_to_echarts_test.py passes (pre-existing processor test).
• npm run test -- plugins/legacy-preset-chart-nvd3 passes.
• Manual: superset db upgrade then open a formerly-legacy bubble chart — renders as bubble_v2; superset db downgrade restores the original form_data from the backup.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• 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: proportional to the number of saved bubble slices (same paginated_update pattern as prior viz migrations); no downtime expected
• Introduces new feature or API
• Removes existing feature or API

🤖 Generated with Claude Code

[bito-code-review]

Bito Automatic Review Skipped - Branch Excluded

Bito didn't auto-review because the source or target branch is excluded from automatic reviews.
No action is needed if you didn't intend for the agent to review it. Otherwise, to manually trigger a review, type /review in a comment and save.
You can change the branch exclusion settings here, or contact your Bito workspace admin at evan@preset.io.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.70%. Comparing base (907b18a) to head (a3cf099).
⚠️ Report is 6 commits behind head on remove-legacy-viz-pipeline.

Additional details and impacted files

@@                      Coverage Diff                       @@
##           remove-legacy-viz-pipeline   #41728      +/-   ##
==============================================================
+ Coverage                       63.91%   64.70%   +0.79%     
==============================================================
  Files                            2697     2695       -2     
  Lines                          148702   148647      -55     
  Branches                        34331    34324       -7     
==============================================================
+ Hits                            95040    96188    +1148     
+ Misses                          51912    50699    -1213     
- Partials                         1750     1760      +10     

Flag	Coverage Δ
hive	39.20% <100.00%> (+0.01%)	⬆️
javascript	69.52% <ø> (+<0.01%)	⬆️
mysql	57.85% <100.00%> (?)
postgres	57.91% <100.00%> (?)
presto	40.75% <100.00%> (+0.01%)	⬆️
python	59.30% <100.00%> (+1.66%)	⬆️
sqlite	57.49% <100.00%> (+<0.01%)	⬆️
unit	100.00% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.