fix(studio): update WebMCP test for polyfill fallback

[miga-heygen]

Summary

• The useStudioAgentTools test "registers nothing when the browser has no WebMCP" asserted document.modelContext was absent after mount
• Since feat(studio): fall back to a WebMCP polyfill where the browser has none #3514 added the @mcp-b/global polyfill, the hook installs document.modelContext even without native browser support — that's the polyfill's purpose
• Updated the test to verify that mounting doesn't throw (the real contract), without asserting polyfill absence

This fixes the CI / Test failure on main.

Other main CI failures (not ours)

• Docs / Validate docs — npm error notarget No matching version found for mint@4.2.837 (docs tool dependency)
• Windows / Tests on windows-latest — ECONNREFUSED ::1:3000 (Windows network test infra)
• regression — cancelled, not failing

Test plan

• All 9 useStudioAgentTools tests pass with NODE_ENV=development
• Pre-commit hooks: lint, format, fallow, typecheck all green

— Miga

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