Core default budget values

The policy implementation follows.

modelRunnerDefaults = Object.freeze({
  maxCalls: 8,
  callTimeoutMs: 30000,
  maxInputChars: 32000,
  maxOutputChars: 16000,
  maxSchemaChars: 16000,
  maxOutputTokens: 1024,
  stage: 'model',
  children: Object.freeze({ maxDepth: 1, maxConcurrentCalls: 1, maxChildren: 32 }),
})