feat(theme): allow deployments to register extra theme tokens

[gabotorresruiz]

SUMMARY

Superset's theme system supports custom tokens beyond Ant Design's (for example brandLogoUrl, echartsOptionsOverrides, and the label* / button* tokens in SupersetSpecificTokens). Deployments, white-label distributors, and custom chart plugins legitimately define their own tokens and consume them via useTheme. Today the theme editor validates token names against a fixed allowlist (antdTokenNames.ts), so any such deployment-specific token is flagged as an "Unknown token" warning with no supported way to register it.

This adds an EXTRA_THEME_TOKENS config that lets a deployment register extra custom token names. It follows the exact pattern already used for EXTRA_CATEGORICAL_COLOR_SCHEMES / EXTRA_SEQUENTIAL_COLOR_SCHEMES: a config default, shipped to the frontend in the common bootstrap payload, and merged into the theme editor's valid-token set at runtime.

TESTING INSTRUCTIONS

1. Set EXTRA_THEME_TOKENS = ["myBrandToken"] in superset_config.py.
2. Open the theme editor and add "myBrandToken": "..." inside a theme's token object.
3. Confirm no "Unknown token" warning is shown for it (an unregistered token like "foo" still warns).

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 66.37%. Comparing base (fe06ebe) to head (b60e693).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #42874   +/-   ##
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  Coverage   66.37%   66.37%           
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  Files        2857     2857           
  Lines      161048   161053    +5     
  Branches    37046    37049    +3     
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+ Hits       106892   106898    +6     
+ Misses      52141    52139    -2     
- Partials     2015     2016    +1     

Flag	Coverage Δ
hive	38.26% <100.00%> (+<0.01%)	⬆️
javascript	73.20% <100.00%> (+<0.01%)	⬆️
mysql	57.75% <100.00%> (+<0.01%)	⬆️
postgres	57.79% <100.00%> (-0.01%)	⬇️
presto	40.22% <100.00%> (+<0.01%)	⬆️
python	59.19% <100.00%> (-0.01%)	⬇️
sqlite	57.42% <100.00%> (+<0.01%)	⬆️
unit	100.00% <ø> (ø)

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