Add runtime schema definition generation

[CSullivan102]

Summary

Adds an opt-in generateHackRuntimeSchemaDefinition schema flag that generates a static memoized getRuntimeSchemaDefinition(): dict<string, mixed> method on generated validators.

The runtime schema definition is self-contained for consumers that need the schema at runtime without reading or resolving the source JSON schema.

Behavior

The generated runtime definition:

• resolves $ref values
• preserves a local description / desc override on resolved refs
• strips codegen-only metadata
• strips $ref for hackIgnoreRef subtrees to avoid dangling runtime refs
• converts desc to description
• converts draft-04 boolean exclusiveMinimum / exclusiveMaximum to numeric bounds

Tests

• ./vendor/bin/hacktest tests/RuntimeSchemaDefinitionTest.php
• ./vendor/bin/hh-autoload && hh_server --check .
• ./vendor/bin/hacktest tests
• ./vendor/bin/hhast-lint src/Codegen/Codegen.php tests/RuntimeSchemaDefinitionTest.php