feat(mcp): Chart formatting options

[msyavuz]

SUMMARY

Adds an optional color_scheme field to the MCP XYChartConfig and PieChartConfig so LLM clients can pick a Superset color palette when generating charts. Pie's hardcoded
"supersetColors" is now an overridable default; XY had no color control before. Wired through map_xy_config (via a new add_color_scheme helper to keep it under the C901 complexity
limit) and map_pie_config.

Example:

{
  "dataset_id": 1,
  "config": {
    "chart_type": "xy",
    "x": {"name": "ds"}, "y": [{"name": "val", "aggregate": "SUM"}],
    "kind": "line",
    "color_scheme": "lyftColors"
  }
}

This is a minimal first slice of broader MCP chart-formatting work (number/date format, axis/data labels, dashboard-level metric color overrides) — those will follow in separate PRs.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

1. pytest tests/unit_tests/mcp_service/chart/test_chart_utils.py::TestMapXYConfig tests/unit_tests/mcp_service/chart/test_new_chart_types.py::TestMapPieConfig (3 new tests + existing 24
   pass).
2. Via MCP generate_chart: pass "color_scheme": "lyftColors" in an XY or Pie config and confirm it lands in form_data.color_scheme; omit it on XY and confirm the key is absent (Superset
   default applies).

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
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.36%. Comparing base (d618837) to head (0b97cc5).

Files with missing lines	Patch %	Lines
superset/mcp_service/chart/chart_utils.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #39887      +/-   ##
==========================================
- Coverage   64.36%   64.36%   -0.01%     
==========================================
  Files        2569     2569              
  Lines      134745   134751       +6     
  Branches    31278    31279       +1     
==========================================
+ Hits        86731    86734       +3     
- Misses      46516    46519       +3     
  Partials     1498     1498              

Flag	Coverage Δ
hive	39.67% <50.00%> (+<0.01%)	⬆️
mysql	59.93% <50.00%> (-0.01%)	⬇️
postgres	60.02% <50.00%> (-0.01%)	⬇️
presto	41.42% <50.00%> (+<0.01%)	⬆️
python	61.55% <50.00%> (-0.01%)	⬇️
sqlite	59.64% <50.00%> (-0.01%)	⬇️
unit	100.00% <ø> (ø)

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