fix: correct type error in themes test (pass ThemeId not ThemeDefinition)

[tmdeveloper007]

Closes #2699.

Summary of What Has Been Done: Fixed a TypeScript error in test/themes.test.ts where the getThemeDefinition test was passing a ThemeDefinition object instead of a ThemeId to the function under test. This caused a TS2345 type error in the CI type-check step.

Changes Made:

• test/themes.test.ts: Fixed getThemeDefinition test to pass theme.id (ThemeId) instead of theme (ThemeDefinition)
• test/themes.test.ts: Fixed nextThemeId invalid-input test to use as ThemeId cast
• Added import for ThemeId type for type casting in tests

Impact it Made: Resolves the CI failure on PR #2704. All other checks (Lint, CodeQL, Build, etc.) passed; only the type-check step failed.

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

Branch has conflicts with main due to the themes test file being updated after this PR was opened. Please rebase onto main and resolve conflicts — the fix itself is correct and needed.