fix(symfony): log the ErrorListener diagnostic at debug level

[davidmpaz]

Q	A
Branch?	4.3
Tickets	Closes #8432
License	MIT
Doc PR	n/a

Changes ErrorListener::duplicateRequest() to log its diagnostic at debug level instead of error.

The message announces that the listener is converting an exception into an Error resource — normal operation, not a fault — and the call is already gated behind $this->debug.

The practical effect: without Monolog, Symfony falls back to HttpKernel\Log\Logger, and the phpunit/phpunit recipe in symfony/recipes ships SHELL_VERBOSITY=-1, which sets that logger's threshold to exactly error. So any functional test asserting a 4xx prints the line in the middle of an otherwise green suite:

............................................[error] An exception occured, transforming to an Error resource.
.....................  66 / 66 (100%)

OK (66 tests, 225 assertions)

which reads as a failure. It is also unreachable by framework.exceptions[].log_level, Symfony's own mechanism for reclassifying client errors, because the level is hardcoded here.

Test

testDiagnosticIsLoggedAtDebugLevel asserts debug() is called and error() is not. I verified it fails against the unpatched listener:

1) ApiPlatform\Symfony\Tests\EventListener\ErrorListenerTest::testDiagnosticIsLoggedAtDebugLevel
Psr\Log\LoggerInterface::error('An exception occured, transfo...ource.', [...]): void was not expected to be called.

and passes with it. src/Symfony/Tests/EventListener/ErrorListenerTest.php is green (5 tests, 34 assertions).

Notes

• No BC concern that I can see: the message, its context array and the $this->debug guard are unchanged, only the severity.
• php-cs-fixer reports nothing to fix on either file.
• Running the full src/Symfony/Tests suite, one unrelated failure remains in ApiPlatformExtensionTest (api_platform.swagger_ui.processor). I confirmed it is pre-existing by re-running it on a pristine 4.3 checkout with my changes stashed.
• I left the occured → occurred typo alone to keep this to a single concern. Happy to add it here or in a separate PR, whichever you prefer.