feat(mcp): add series_limit to generate_chart XY config

[aminghadersohi]

SUMMARY

Adds a series_limit parameter to XYChartConfig in the generate_chart MCP tool.

When group_by is set on an XY chart (line, bar, area, scatter), users can now control the maximum number of distinct series rendered by specifying series_limit. The value is forwarded as form_data["series_limit"], which Superset's echarts timeseries plugins use to cap the number of rendered series lines/bars.

Usage example:

{
  "dataset_id": 123,
  "config": {
    "chart_type": "xy",
    "x": {"name": "order_date"},
    "y": [{"name": "revenue", "aggregate": "SUM"}],
    "group_by": [{"name": "region"}],
    "kind": "line",
    "series_limit": 10
  }
}

When series_limit is not set (default None), the field is omitted from form_data and Superset applies no series cap.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A — backend schema change only.

TESTING INSTRUCTIONS

1. Call generate_chart with an XY config that includes group_by and series_limit: 5
2. Verify the returned form_data contains "series_limit": 5
3. Open the explore URL — the chart should show at most 5 series
4. Call without series_limit — verify form_data does not contain series_limit

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.13%. Comparing base (e25d708) to head (c256076).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
superset/mcp_service/chart/chart_utils.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #40307      +/-   ##
==========================================
- Coverage   64.13%   64.13%   -0.01%     
==========================================
  Files        2592     2592              
  Lines      138907   138912       +5     
  Branches    32217    32218       +1     
==========================================
+ Hits        89085    89087       +2     
- Misses      48290    48293       +3     
  Partials     1532     1532              

Flag	Coverage Δ
hive	39.30% <33.33%> (+<0.01%)	⬆️
mysql	58.81% <33.33%> (-0.01%)	⬇️
postgres	58.89% <33.33%> (-0.01%)	⬇️
presto	40.97% <33.33%> (-0.01%)	⬇️
python	60.45% <33.33%> (-0.01%)	⬇️
sqlite	58.53% <33.33%> (-0.01%)	⬇️
unit	100.00% <ø> (ø)

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