chore(viz)!: remove explore_json endpoints, viz.py and the legacy chart data pipeline

[rusackas]

SUMMARY

Phase 2 of #41714 (targets remove-legacy-viz-pipeline): with all 15 legacy charts migrated to /api/v1/chart/data, this removes the deprecated legacy pipeline at its declared 5.0.0 EOL.

Backend removed:

• /superset/explore_json/ + /superset/explore_json/data/<cache_key> routes, their viz-only response helpers (generate_json, CSV/XLSX/query/results/samples), and the CSRF exemption
• superset/viz.py (−2,000 lines) and every importer: get_viz + explore-cache permission checkers in views/utils, Slice.viz + Slice.explore_json_url, the legacy warm-up-cache path, the legacy annotation-data path in the query context processor (annotation source charts now always resolve through their query context), and the BaseViz overload of security_manager.raise_for_access
• the load_explore_json_into_cache celery task and the async query manager's explore_json job plumbing (GLOBAL_ASYNC_QUERIES chart-data jobs are unaffected)
• the can_explore_json grant in the public-role permission set and the legacy-prefix redirect entries for the removed routes (the middleware itself stays)

Frontend removed:

• ChartMetadata.useLegacyApi and every branch keyed on it: chartAction's legacy request/response handling, exportChart's legacy endpoint, ChartClient/StatefulChart explore_json endpoints, DrillBy legacy handling, and the explore panel's "chart type requires a dataset" alert (only reachable for legacy charts on non-table datasources)
• getURIDirectory now always resolves to /explore/; getLegacyEndpointType/shouldUseLegacyApi helpers and the vendored deck.gl/nvd3 explore-URL utils are gone

Breaking changes (staged in UPDATES.md for UPDATING.md): third-party plugins setting useLegacyApi or importing superset.viz must migrate to buildQuery + the v1 chart data API.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A — removal of an unreachable code path (no registered chart routes to it anymore).

TESTING INSTRUCTIONS

• pytest tests/unit_tests/ passes (402 tests across the touched modules re-verified; legacy-pipeline tests removed or retargeted).
• npx jest src/explore src/components/Chart packages/superset-ui-core — 196 tests pass.
• pre-commit run --all-files green (mypy/ruff/pylint/tsc/eslint).
• Manual: no chart issues POST /superset/explore_json/ anywhere in the app; exports, drill-by, annotations and cache warm-up all flow through v1.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
• Introduces new feature or API
• Removes existing feature or API

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (remove-legacy-viz-pipeline@b60cf4e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...tend/src/components/Chart/DrillBy/DrillByModal.tsx	50.00%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##             remove-legacy-viz-pipeline   #41750   +/-   ##
=============================================================
  Coverage                              ?   64.80%           
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  Files                                 ?     2700           
  Lines                                 ?   147243           
  Branches                              ?    34116           
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  Hits                                  ?    95420           
  Misses                                ?    50142           
  Partials                              ?     1681           

Flag	Coverage Δ
hive	39.52% <37.50%> (?)
javascript	69.61% <91.66%> (?)
mysql	57.76% <100.00%> (?)
postgres	57.82% <100.00%> (?)
presto	41.10% <37.50%> (?)
python	59.25% <100.00%> (?)
sqlite	57.39% <100.00%> (?)
unit	100.00% <ø> (?)

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