fix(tests): Ensure ThemeFixture returns valid theme object for ThemeProvider

[sentry]

This PR addresses a TypeError: Invalid value used as weak map key occurring in test environments, specifically within customRender in reactTestingLibrary.tsx.

Problem:
The error originated from @emotion/weak-memoize (used by @emotion/react's ThemeProvider) when the theme prop passed to ThemeProvider was not a valid object. This became a critical issue after the React 19 / Emotion 11.14 upgrade, as weak-memoize became stricter, throwing an error for non-object WeakMap keys (e.g., undefined or primitives).

In our test setup, customRender uses <ThemeProvider theme={ThemeFixture()}>. The ThemeFixture() function was returning lightTheme which, under certain module resolution timings or circular import scenarios, could resolve to undefined or a non-object value at the point it was used by ThemeProvider.

Solution:

1. tests/js/fixtures/theme.tsx: Modified ThemeFixture to include a runtime check. It now explicitly verifies that lightTheme is a non-null object. If not, it throws a more descriptive error, preventing the opaque WeakMap error. It also returns a spread copy ({...lightTheme}) to ensure a fresh, plain object is always provided, preventing accidental mutations and ensuring compatibility with weak-memoize.
2. tests/js/sentry-test/reactTestingLibrary.tsx: In customRender, the result of ThemeFixture() is now explicitly stored in a theme variable and validated to be a non-null object before being passed to <ThemeProvider>. This adds an additional layer of safety and provides an earlier, clearer error message if ThemeFixture somehow returns an invalid value.

These changes ensure that ThemeProvider always receives a valid object as its theme prop in tests, resolving the TypeError and making the test harness more robust against theme resolution issues.

Legal Boilerplate

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Fixes JEST-2KVF

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📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.56%