fix(workflow): prevent concurrent map read/write panic during stream processing

[phpoh]

Summary

• Prevents "concurrent map iteration and map write" panics during workflow stream processing
• Removes single-chunk shortcuts that returned shared map references directly
• Always uses ConcatMaps to create deep copies of map data

Problem

When processing streaming output chunks, the code directly used shared map references without deep copying. This caused "concurrent map iteration and map write" panics when other goroutines were still modifying the original maps during JSON marshaling.

The panic occurred in mustMarshalToString when serializing event.Output while the original map was still being modified elsewhere.

Changes

1. In buildStreamEndEvent: Removed single-chunk shortcut that returned shared map reference directly
2. In ConcatStructuredCallbackOutputs: Removed single-element shortcuts for inputList and extraList
3. In ConcatStructuredCallbackInputs: Removed single-input shortcut and extraList shortcut

All map data is now processed through ConcatMaps which creates deep copies, preventing concurrent access issues.

Test plan

• Build passes: go build ./domain/workflow/internal/execute/... ./domain/workflow/internal/nodes/...
• Unit tests pass for the modified packages
• Manual testing with concurrent workflow execution (recommended by reviewer)

Fixes #2607

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