Fix action cache extract

[Dingway98]

Note

Tightens instruction/objective handling in action cache and replay, adding validation for extract and safer fallbacks when instructions are missing.

• Agent runtime (src/agent/index.ts):
  • Validate extract replay: require step.instruction; fail with clear error if missing; wrap extraction in try/catch with error reporting.
  • perform helpers: pass performInstruction as null when absent; if no valid helper and no instruction, return FAILED with explanatory message.
• Action cache (src/agent/shared/action-cache.ts):
  • Refactor extractInstruction to return objective for extract, require instruction for actElement, and allow undefined for actions that don't need instructions.
• Types (src/types/agent/types.ts):
  • Change ActionCacheEntry.instruction to string | undefined to reflect optional instructions.

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