[Server] Flush output buffers on stateless SSE streams

[SammyTourani]

Fixes #516

Fixes #516.

• src/Server/Transport/Http/StatelessResponder.php: call @ob_flush() before
  flush() after each frame, the same two calls StreamableHttpTransport
  already makes. The @ suppresses the notice ob_flush() raises when no
  output buffer is active.
• tests/Integration/StatelessLifecycleTest.php:
  • The example server is now started with -d output_buffering=4096. Without
    that, whether it buffers depends on the php.ini the machine loads.
  • The new testListenIsAcknowledgedAtOnce opens subscriptions/listen and
    requires the first data: frame to be the acknowledgement. It reads with a
    5 s timeout, well inside the example's 20 s lifetime.

Verification

Command: vendor/bin/phpunit --testsuite=unit && vendor/bin/phpunit --testsuite=integration

Result (PHP 8.4.23):

OK (1582 tests, 4107 assertions)
OK (69 tests, 156 assertions)

• The new test, without the fix: it fails after the 5 s read timeout with
  Failed asserting that null is identical to 'notifications/subscriptions/acknowledged'.
  With the fix it passes in 0.07 s.
• PHP 8.1.34: the new test likewise fails without the fix and passes with
  it. tests/Integration/StatelessLifecycleTest.php gives
  OK (11 tests, 39 assertions) and the unit suite gives
  OK (1582 tests, 4107 assertions).
• Manual check: I ran the stateless-lifecycle example under
  php -d output_buffering=4096 -S … and timed each SSE line of a
  subscriptions/listen stream over a raw socket.
  • Before the fix, the acknowledgement arrived at 20.22 s, together with the
    closing result.
  • After the fix, it arrived at 0.01 s.
  • With output_buffering=0, both versions deliver it at 0.01 s.
• Client hanging up after 0.5 s (same setup): without the fix, the single
  php -S worker stayed busy for another 19.59 s. With the fix it answered the
  next request 0.02 s later.
• Style and static analysis: php-cs-fixer fix --dry-run --diff on both
  files found 0 of 2 files to fix, and phpstan reported no errors.