[v3-2-test] Allow UI theme config without tokens (CSS-only or empty fallback) (#64552)

[github-actions]

• Add .worktrees/ to .gitignore

• Make Theme tokens field optional to allow CSS-only or empty theme configs

• Add integration test: CSS-only theme passes config endpoint validation

• Docs: note that Theme tokens is optional, empty config restores OSS defaults

• Improve CSS-only theme test: also assert icon fields absent from response

• Add newsfragment for optional theme tokens

• Rename newsfragment to PR Allow UI theme config without tokens (CSS-only or empty fallback) #64552

• Fix TypeScript errors in createTheme for optional tokens support

The API schema for Theme became opaque ({[key: string]: unknown}) after
@model_serializer was added to Theme in Python to exclude None values.
This broke theme.ts in two ways:

• userTheme.tokens typed as unknown, not assignable to TokenDefinition
• mergeConfigs() called with undefined, which it doesn't accept

Fix by conditionally spreading tokens/globalCss (guarding for CSS-only
themes) and casting to Record<string, unknown> which is assignable to
TokenDefinition. Use conditional mergeConfigs call to avoid passing
undefined.

• Fix Theme OpenAPI schema collapsing to untyped object

The @model_serializer(mode="wrap") on Theme and ThemeColors caused
Pydantic to generate additionalProperties:true for both models in the
OpenAPI spec, losing typed fields (tokens, globalCss, icon) and
breaking TypeScript types to {[key: string]: unknown}.

Fix: remove wrap serializers from Theme/ThemeColors; add
@field_serializer("theme") to ConfigResponse with model_dump(exclude_none=True)
to preserve None-exclusion behavior scoped to the theme field only.
Add ConfigDict(json_schema_mode_override="validation") to ConfigResponse
so schema generation uses type annotations rather than the field
serializer's dict return type, keeping the $ref: Theme link intact.

Also removes unrelated .worktrees/ entry from .gitignore.
(cherry picked from commit a06896f)

Co-authored-by: Constance Martineau constance.alyssa.martineau@gmail.com