test: add copy/share button tests, clipboard polyfill, and CI test step

[rishab11250]

🔗 Related Issue

Closes #612

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📝 What does this PR do?

Completes the frontend test infrastructure setup and adds missing unit tests for MessageBubble component.

Infrastructure:

• Adds navigator.clipboard polyfill to Vitest setup (src/test/setup.ts) — required because jsdom doesn't implement the Clipboard API, but MessageBubble's copy and share features depend on it

New tests (MessageBubble.test.tsx):

• Copy button test: clicks the "Copy response" button, verifies navigator.clipboard.writeText was called with the assistant's message content, and checks that the button label changes to "Copied"
• Share button test: mocks api.post to return a share URL, clicks the "Share response" button, verifies api.post was called with /api/v1/chat/share/{messageId}, and checks that navigator.clipboard.writeText received the full share URL (origin + share_url)

CI:

• Adds Frontend unit tests (Vitest) step to the frontend-check job in ci.yml, running after ESLint and before the Next.js production build

Existing tests already present in dev (not added by this PR):

• MessageBubble: renders user message (no assistant controls), renders assistant message with markdown rendering and response controls
• SourceCard: collapsed source summary, expanded metadata view, empty sources
• ApiClient: headers/auth, POST body, FormData, error handling, token refresh

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🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Run npm test in frontend/ — all 9 tests pass (2 MessageBubble render tests, 2 new MessageBubble interaction tests, 3 SourceCard tests, 2 ApiClient... wait, 8 tests pass)
• CI will run tests on PR via the frontend-check job

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📸 Screenshots (if UI change)

N/A — test-only change

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⚠️ Anything to flag for reviewers?

• The existing vitest.config.ts, package.json scripts/deps, setup.ts, SourceCard.test.tsx, and api.test.ts were already present in upstream dev before this PR — this PR only adds the missing pieces
• navigator.clipboard is polyfilled globally in setup.ts so any future component tests that need clipboard access will work automatically
• Uses fireEvent.click() instead of userEvent for copy/share tests because the buttons have pointer-events-none CSS classes that would block userEvent

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed