test: stabilize modal context pricing test

[escape0707]

Summary

Fixes #36523

Mock the lazy-loaded billing Pricing component in modal-context.test.tsx so the provider test verifies the modal transition without racing the full pricing page dynamic import.

Validation:

• pnpm -C web test context/modal-context.test.tsx --run
• pnpm -C web exec eslint context/modal-context.test.tsx --quiet

Screenshots

N/A

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods