fix(studio): drop the resurrected WebMCP polyfill assertion

[miguel-heygen]

main is going red at random for open PRs on one line in
packages/studio/src/webmcp/useStudioAgentTools.test.tsx.

da6514d45 removed expect(document).not.toHaveProperty("modelContext") after
the @mcp-b/global polyfill fallback landed. #3516 merged from a branch cut
before that commit and brought the line back, together with the old test title.
This re-applies da6514d45's change to that one case; nothing else moves.

Why it is flaky rather than just wrong

useStudioAgentTools awaits a dynamic import("@mcp-b/global") inside its
mount effect. Whether document.modelContext exists when the assertion runs
depends only on whether that import has resolved yet, and await act()
usually returns first. main won that race and went green; another PR's run
lost it and went red.

Made it deterministic on the current tip by inserting a 100ms wait before the
assertion:

 ❯ src/webmcp/useStudioAgentTools.test.tsx:191:26
     191|     expect(document).not.toHaveProperty("modelContext");
 Test Files  1 failed (1)
       Tests  1 failed | 8 passed (9)

The failure dump shows all five tools registered through the polyfill, so the
real behaviour is the opposite of what the line asserted. Installing
document.modelContext where the browser has none is exactly the polyfill's
job. What the case is actually for is that mounting does not throw, so a
browser without native WebMCP still boots Studio.

Checked for other resurrections

3337cc899 is a squash with a single parent, so its diff is its full net
change to main. Reviewed every hunk in the four files that existed before it
(StudioAgentTools.tsx, useStudioAgentTools.ts, useStudioAgentTools.test.tsx,
tools/selectionTools.test.ts). Only this one hunk reverses earlier work; the
rest are additive, and the selectionTools.test.ts shrink is a helper
extraction into the new webmcpTestUtils.ts. The #3514 polyfill code in
useStudioAgentTools.ts is untouched.

Verification

• Target file run 10x: 10/10 green, 9 tests each.
• Same 100ms probe re-applied on top of this change: still 9 passed. The test
  no longer depends on who wins the race.
• Full studio suite: 412 passed | 1 skipped (413) files,
  4591 passed | 18 todo (4609) tests.
• oxfmt --check and oxlint clean on the changed file; pre-commit lint,
  fallow and typecheck gates all passed.