fix: prevent group_data override in MAS and add missing replanner_agent_name

[Takki-cd]

Summary

Fix two bugs in OxyGent framework: group_data being unexpectedly overwritten in MAS and missing replanner_agent_name field in PlanAndSolve flow.

Changes

1. MAS (mas.py) - Fix group_data override issue

• Problem: When from_trace_id is provided, the framework correctly merges historical group_data from ES with current group_data. However, a subsequent loop overwrites the merged result with the raw payload value, This causes historical context to be lost when other agents or
  frontend pass their own group_data.
• Solution: Reorder the code to set all payload fields first, then perform the special group_data merge logic. This ensures the merged result is the final value.

2. PlanAndSolve (plan_and_solve.py) - Add missing replanner_agent_name

• Problem: replanner_agent_name is used in the code when enable_replanner=True, but the field was never defined, causing AttributeError.
• Solution: Add the missing field definition with default value "replanner_agent".

Impact

• Users can now safely pass group_data in multi-agent scenarios without losing historical context
• enable_replanner feature in PlanAndSolve will work correctly