(4/9) Reject request-only APIs in output: 'export' fallback pages

[feedthejim]

Context

Exported fallback pages still render at build time, so values that only exist per request cannot work there.

What This PR Does

• errors on unresolved fallback params in Server Components
• errors on request-only APIs like headers(), cookies(), connection(), and searchParams in export fallback rendering
• covers both direct and suspense-wrapped cases in the export e2e fixtures

Why This Is Separate

This isolates the policy around request-derived values. Reviewers can look at that rule by itself before the next PR handles the separate question of time, randomness, and use cache.

Not In This PR

• sync IO rules such as Date.now() and Math.random()
• hard-load bootstrap UX
• prefetch and hydration reuse
• not-found recovery

Validation

• pnpm --filter=next build
• NEXT_SKIP_ISOLATE=1 NEXT_TEST_MODE=start pnpm testheadless test/e2e/app-dir-export/test/dynamic-fallback-server-params.test.ts test/e2e/app-dir-export/test/dynamic-fallback-server-params-suspense.test.ts test/e2e/app-dir-export/test/output-export-server-connection.test.ts test/e2e/app-dir-export/test/output-export-server-connection-suspense.test.ts test/e2e/app-dir-export/test/output-export-server-cookies.test.ts test/e2e/app-dir-export/test/output-export-server-cookies-suspense.test.ts test/e2e/app-dir-export/test/output-export-server-headers.test.ts test/e2e/app-dir-export/test/output-export-server-headers-suspense.test.ts test/e2e/app-dir-export/test/output-export-server-search-params.test.ts test/e2e/app-dir-export/test/output-export-server-search-params-suspense.test.ts

Stack

Part 4 of 9 in the output-export fallback stack.

1. #93032 (1/9) Add the basic runtime for app-router fallbacks in output: 'export'``
2. #93025 (2/9) Cover output: 'export' fallbacks across app-router route shapes
3. #93026 (3/9) Prefer the most specific route for overlapping export fallbacks
4. #93033 (4/9) Reject request-only APIs in output: 'export' fallback pages (this PR)
5. #93027 (5/9) Define sync IO rules for output: 'export' fallback pages
6. #93028 (6/9) Hide the export fallback bootstrap document on hard loads
7. #93029 (7/9) Prefetch and dedupe export fallback data requests
8. #93030 (8/9) Reuse learned export fallback paths after hydration
9. #93031 (9/9) Use app not-found when no export fallback route matches

[github-actions]

All broken links are now fixed, thank you!