docs(power): use class references instead of strings in retryableErrorTypes examples

[TrickSumo]

Issue

The retryableErrorTypes examples in error-handling.md and step-operations.md pass string literals instead of class references:

retryableErrorTypes: ['NetworkError', 'TimeoutError'] // ❌ broken

The SDK type signature is retryableErrorTypes?: (new () => Error)[] which requires class constructors, not strings. Passing strings causes a TypeScript compile error:

  Type 'string' is not assignable to type 'new () => Error'

Description of changes:

Replace string literals with class references

retryableErrorTypes: [NetworkError, TimeoutError] // ✅ correct