docs(mcp): document all 7 generate_chart chart_type values, fix stale config comment

[aminghadersohi]

SUMMARY

The generate_chart MCP tool's docstring only documented chart_type='xy' and chart_type='table', even though the underlying schema (ChartConfig discriminated union) has supported 5 additional chart types since #38375, #38402, and #38403: pie, pivot_table, mixed_timeseries, handlebars, and big_number.

Because the docstring is the primary signal an LLM client sees before calling the tool, models reaching for natural chart-type names like 'line', 'bar', or 'pie' directly as the chart_type value (instead of as a kind sub-field of chart_type='xy', or simply being unaware 'pie' was a valid discriminator at all) would hit the raw Pydantic discriminator validation error and loop through retries until timing out.

This PR only touches docstrings/comments — no schema, validation, or behavior changes:

• generate_chart docstring now lists all 7 valid chart_type values with their required fields
• Clarifies that kind (line/bar/area/scatter) is a sub-field of chart_type='xy', not a chart_type itself
• Adds a natural-language → chart_type lookup table
• Adds a pie chart example (previously only xy and table had examples)
• Fixes a stale comment in MCP_TOOL_SEARCH_CONFIG that described include_schemas=False as the default; it has defaulted to True since fix(mcp): restore typed ChartConfig in tool schemas for LLM visibility #39732 (to keep discriminated-union chart configs visible to LLMs without a second round trip)

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A — docstring/comment-only change, not user-facing UI.

TESTING INSTRUCTIONS

1. ruff format --check / ruff check pass on both changed files
2. Pre-commit hooks (mypy, ruff, pylint, docstring-first check) pass
3. No tests assert on the literal docstring/comment text; existing tests/unit_tests/mcp_service/chart/tool/test_generate_chart.py suite is unaffected since only documentation (not schema or logic) changed

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.42%. Comparing base (35194fe) to head (4ce61b7).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #41599      +/-   ##
==========================================
- Coverage   64.42%   64.42%   -0.01%     
==========================================
  Files        2668     2668              
  Lines      147182   147182              
  Branches    33947    33947              
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- Hits        94821    94816       -5     
- Misses      50646    50649       +3     
- Partials     1715     1717       +2     

Flag	Coverage Δ
hive	39.10% <ø> (ø)
mysql	57.65% <ø> (ø)
postgres	57.72% <ø> (-0.01%)	⬇️
presto	40.65% <ø> (ø)
python	59.13% <ø> (-0.01%)	⬇️
sqlite	57.35% <ø> (ø)
unit	100.00% <ø> (ø)

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