Fix Mobile-Agent-v3 action parsing for Python-style dict output

[vwww-droid]

Summary

• Add a fallback parser in run_mobileagentv3.py to handle Python-dict style action strings returned by some OpenAI-compatible models.
• Reuse the parsed action_object throughout the step loop instead of re-decoding raw action text, preventing secondary JSONDecodeError crashes.
• Keep behavior unchanged for valid JSON action payloads.

Test plan

• Run python3 -m py_compile Mobile-Agent-v3/mobile_v3/run_mobileagentv3.py.
• Reproduce local run where action payload uses single quotes and confirm the loop continues past operator/reflection.
• Validate one end-to-end UI task on-device after merging.

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