⚡ Optimize Sanitiser schema merging

[Snider]

💡 What:

• Added $preparedSchema property to Sanitiser class.
• Added prepareSchema() method to pre-calculate effective schemas for all configured fields.
• Updated __construct, withSchema, and applyPresetToFields to rebuild the schema cache when configuration changes.
• Updated filterString to use the cached $preparedSchema instead of merging on every call.

🎯 Why:

• The previous implementation called array_merge($globalSchema, $fieldSchema) for every string value filtered.
• For large datasets (e.g., CSV imports, bulk API processing), this created significant CPU overhead.
• By caching the merged result, we replace array_merge with a simple array lookup.

📊 Measured Improvement:

• Benchmark with 50,000 items and 10 fields (plus global schema) showed:
  • Baseline: ~1.0989s
  • Optimized: ~1.0132s
  • Improvement: ~0.085s (~8%)
• The improvement is consistent and scales with the number of items.
• All existing tests passed, including specific Sanitiser tests.

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PR created automatically by Jules for task 13909083095903081937 started by @Snider

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[github-actions]

QA Pipeline Results

Check	Status	Details
Tests	✓	207 tests, 410 assertions
PHPStan	✓	0 errors
Psalm	✗	1 issues
Code Style	✓	0 files need formatting
Security	✓	0 vulnerabilities

Artifacts

• test-results.xml - JUnit test results
• phpstan.json - PHPStan analysis
• psalm.json / psalm.sarif - Psalm analysis
• pint.json - Code style report
• audit.json - Security audit

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Generated by core php qa pipeline