test(ui): guard the Cloudflare build's self-heal step against regression

[JSONbored]

Summary

Closes #6642.

• CI's UI build step doesn't reproduce Cloudflare Pages' actual build invocation #6642 asked for a CI check exercising apps/loopover-ui's build:cloudflare script (scoped to that directory) so a future regression of fix(ui): self-heal the Cloudflare Pages build's incomplete workspace install #6624's class is caught before merge.
• I tested that approach empirically first: a plain npm ci run from apps/loopover-ui in a normal git checkout correctly finds and installs the full workspace tree (confirmed live — it reaches for the root node_modules/sharp and attempts to build it). Cloudflare's actual build sandbox does something else to produce its partial 957-package install, which isn't reproducible via plain npm/git commands in GitHub Actions. A CI job that just cds into apps/loopover-ui and runs npm ci would always pass, giving false confidence rather than a real regression test.
• The reliable, cheap invariant that actually prevents this regression: build:cloudflare must keep running a full monorepo-root npm ci before ui:build (that's what fix(ui): self-heal the Cloudflare Pages build's incomplete workspace install #6624 added). This adds a unit test asserting that exact script shape, mirroring the existing test/unit/ci-ui-build-openapi.test.ts convention of testing critical build-recipe content directly rather than trying to functionally simulate an external platform's build sandbox.
• Verified the test actually catches the regression: reverted build:cloudflare locally to the pre-fix(ui): self-heal the Cloudflare Pages build's incomplete workspace install #6624 shape, confirmed the test goes red, restored it.

Scope

• test/unit/ui-cloudflare-build-self-heal.test.ts only (new file)
• No src/** changes

Validation

• npx vitest run test/unit/ui-cloudflare-build-self-heal.test.ts — passes
• Manually reverted build:cloudflare's content and re-ran — test fails as expected, confirming it's a real guard
• npx vitest run (full unsharded suite) — 930/932 files, 17826/17839 tests passed (pre-existing skips unchanged)
• npx tsc --noEmit — clean

Safety

• No secrets touched

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.64%. Comparing base (a47d1c3) to head (db8f3ae).
⚠️ Report is 9 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6648   +/-   ##
=======================================
  Coverage   93.64%   93.64%           
=======================================
  Files         680      680           
  Lines       68026    68026           
  Branches    18673    18673           
=======================================
  Hits        63704    63704           
  Misses       3347     3347           
  Partials      975      975           

Flag	Coverage Δ
shard-1	43.93% <ø> (-0.09%)	⬇️
shard-2	36.99% <ø> (+0.21%)	⬆️
shard-3	32.41% <ø> (-0.01%)	⬇️
shard-4	33.76% <ø> (-1.01%)	⬇️
shard-5	31.62% <ø> (+0.64%)	⬆️
shard-6	45.78% <ø> (+0.07%)	⬆️

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