fix(planner): handle concatenated plan responses

[March-77]

Summary

• detect when the planner model emits multiple complete JSON root objects in one response
• use the final complete plan so model self-corrections are not silently ignored
• preserve the existing converter path for normal single-plan responses and wrapped output

Validation

• ./mvnw -pl data-agent-management -Dtest=PlanProcessUtilTest test (13 tests)
• ./mvnw -pl data-agent-management test (1630 tests)

Fixes #275