feat(calendar): migrate cal_heatmap chart to v1 chart data API

[rusackas]

SUMMARY

Tier 2 of #41714 (targets remove-legacy-viz-pipeline): migrate the Calendar Heatmap (cal_heatmap) off explore_json onto /api/v1/chart/data.

• New buildQuery.ts mirroring the legacy CalHeatmapViz.query_obj: timeseries query with the time grain forced from subdomain_granularity (min→PT1M, …, year→P1Y).
• The get_data reshape is ported to transformData: per-metric value maps keyed by unix seconds, and the cal-heatmap domain range computed from the query's from_dttm/to_dttm (which the v1 response carries) using calendar arithmetic that matches dateutil.relativedelta semantics, including month-end clamping. Missing time bounds throw the same "Please provide both time bounds" error the backend raised.
• useLegacyApi: true removed.
• No DB migration: viz_type unchanged; legacy granularity_sqla/time_range handled by extractExtras.

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-calendar — 10 new Jest tests (grain mapping incl. default, unix-second keying, year/month/week/day/hour range arithmetic, relativedelta month-end clamping, missing-bounds error); 17 total pass.
• Manual: open a saved Calendar Heatmap; network tab shows POST /api/v1/chart/data; calendar layout and cell 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 88.46154% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.09%. Comparing base (e318607) to head (43d729f).
⚠️ Report is 1 commits behind head on remove-legacy-viz-pipeline.

Files with missing lines	Patch %	Lines
...legacy-plugin-chart-calendar/src/transformProps.ts	0.00%	4 Missing ⚠️
...ins/legacy-plugin-chart-calendar/src/buildQuery.ts	83.33%	1 Missing ⚠️
.../plugins/legacy-plugin-chart-calendar/src/index.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           remove-legacy-viz-pipeline   #41724   +/-   ##
===========================================================
  Coverage                       64.08%   64.09%           
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  Files                            2687     2689    +2     
  Lines                          148467   148519   +52     
  Branches                        34262    34274   +12     
===========================================================
+ Hits                            95147    95194   +47     
- Misses                          51558    51563    +5     
  Partials                         1762     1762           

Flag	Coverage Δ
javascript	69.44% <88.46%> (+0.01%)	⬆️

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