[Server] Log expected tool failures at debug level

[ousamabenyounes]

Summary

• log expected ToolCallException failures at debug severity instead of error severity
• keep unexpected exceptions at error severity
• preserve and verify the existing log message and context

Fixes #157

Test verification (RED → GREEN)

Before the production change, the focused regression test failed with:

Expected level: debug
Actual level: error
FAILURES! Tests: 1, Assertions: 1, Failures: 1.

After the change:

OK (26 tests, 125 assertions)

Full local CI replay:

PHP CS Fixer: 0 files can be fixed
PHPStan: No errors
PHPUnit: OK (1247 tests, 3396 assertions)
phpDocumentor: complete
php-diff-coverage: PASS 1/1 changed executable lines covered