[fix] resolve env placeholders in post-processor binary validation

[wtorsi]

Summary

• validatePostProcessorBinaries() called is_executable() on raw config values, which at compile time are Symfony env placeholder UUIDs (not actual paths)
• Now uses ContainerBuilder::resolveEnvPlaceholders() to detect env references and resolves them from $_ENV/$_SERVER/getenv() before validating
• Skips validation gracefully when the env var is not available at compile time (runtime-only resolution)

Test plan

• 6 new unit tests in ChamberOrchestraImageExtensionTest covering:
  • Env placeholder resolves to executable binary — passes
  • Env placeholder resolves to non-executable — throws
  • Env var not set at compile time — skips validation
  • Literal executable path — passes
  • Literal non-executable path — throws
  • Unused post-processor binary — skips validation
• Full test suite passes (347 tests, 675 assertions)
• Code style clean (php-cs-fixer dry-run: 0 issues)

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