tests: Add unittest for SchemaEnumAdd, SchemaEnumRemove

[Dhanus3133]

Why

Unittests for SchemaEnumAdd, SchemaEnumRemove mutations were missing.

Closes #1553

What changed

Added tests for testing the SchemaEnumAdd & SchemaEnumRemove

How to test

uv run pytest backend/tests/component/graphql/mutations/test_schema.py

Checklist

• Tests added/updated
• Changelog entry added (uv run towncrier create ...)
• External docs updated (if user-facing or ops-facing change)
• Internal .md docs updated (internal knowledge and AI code tools knowledge)
• I have reviewed AI generated content

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Summary by cubic

Add unit tests for GraphQL SchemaEnumAdd and SchemaEnumRemove. Tests confirm options are added/removed with ok: true, final enums match expected lists, and the schema cache isn’t mutated in place.

^{Written for commit de5b879. Summary will update on new commits. Review in cubic}

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

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_{Comparing Dhanus3133:task/add-SchemaEnumAdd-SchemaEnumRemove-tests (8fb28d2) with stable (edaeba7)}