feat(paired-t-test): migrate paired_ttest chart to v1 chart data API

[rusackas]

SUMMARY

Tier 2 of #41714 (targets remove-legacy-viz-pipeline): migrate the Paired t-test Table (paired_ttest) off explore_json onto /api/v1/chart/data.

• New buildQuery.ts mirroring the legacy PairedTTestViz.query_obj: timeseries query (is_timeseries: true) grouped by the group columns, sort metric appended to the select list, orderby under order_desc, residual order_by_cols ignored.
• The pandas pivot_table reshape from get_data is ported to a transformData helper: one series per (metric, group tuple), {x, y} points on the union of timestamps with missing combinations padded with null, group order matching the sorted pandas pivot columns, All group when ungrouped.
• useLegacyApi: true removed (and the "Legacy" tag dropped).
• No DB migration: viz_type unchanged; legacy granularity_sqla/time_range handled by extractExtras (covered by test).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

No visual change — same renderer, same data shape, different endpoint.

TESTING INSTRUCTIONS

• npm run test -- plugins/legacy-plugin-chart-paired-t-test — 7 Jest tests (timeseries query shape, sort-metric append, orderby ownership, null padding, multi-metric/multi-group nesting, timestamp ordering, All group).
• Manual: open a saved Paired t-test Table; network tab shows POST /api/v1/chart/data; p-values/lift values identical.

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 88.52459% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.63%. Comparing base (c6242a6) to head (4583851).
⚠️ Report is 5 commits behind head on remove-legacy-viz-pipeline.

Files with missing lines	Patch %	Lines
...y-plugin-chart-paired-t-test/src/transformProps.ts	0.00%	5 Missing ⚠️
...ins/legacy-plugin-chart-paired-t-test/src/index.ts	0.00%	1 Missing ⚠️
...cy-plugin-chart-paired-t-test/src/transformData.ts	97.56%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                       @@
##           remove-legacy-viz-pipeline   #41721      +/-   ##
==============================================================
+ Coverage                       64.62%   64.63%   +0.01%     
==============================================================
  Files                            2684     2686       +2     
  Lines                          148431   148491      +60     
  Branches                        34249    34264      +15     
==============================================================
+ Hits                            95930    95984      +54     
- Misses                          50739    50745       +6     
  Partials                         1762     1762              

Flag	Coverage Δ
javascript	69.41% <88.52%> (+0.01%)	⬆️

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