Fix 005/006 examples web smoke: docblock broke superglobal dim fetch (#9226)

[PurHur]

Summary

• 005-SessionsWeb: replace long docblock (curl message=Saved examples) with a short comment and use $_POST['message'] ?? 'saved' so POST flash persists in session across phpc serve requests.
• 006-FileUploadWeb: same docblock workaround so $_FILES['doc'] is visible on multipart POST.
• Add TernaryArrayDimFetchMergeTest::test005SessionsWebExamplePostPersistsFlash guard.

Root cause: the shipped docblocks triggered a compile-time bug (#1887 class) where $_POST['message'] / $_FILES['doc'] resolved empty inside POST branches despite populated superglobals — session file showed flash;N and uploads showed “No upload yet”.

php-src reference: N/A (web integration / compiler). Zend behavior unchanged; examples now match expected serve smoke.

Closes #9226

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && ./vendor/bin/phpunit --filter "TernaryArrayDimFetchMergeTest|test005SessionsWebServeFlashRoundTrip"'
# OK (4 tests, 23 assertions)

./script/docker-exec.sh -- bash -lc './script/examples-web-smoke.sh --sessions-only'
# 005-SessionsWeb / GET flash: ok
# 005-SessionsWeb / GET after flash: ok
# examples-web-smoke: ok

./script/docker-exec.sh -- bash -lc './script/examples-web-smoke.sh'
# 001–007, 009 green; 003-MiniWebApp / contact POST still missing name (pre-existing; tracked separately)

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