fix(adk): preserve empty MCP arrays in task history

[erauner12]

Summary

Preserve nested non-nil empty JSON arrays in MCP structured results as [] when they pass through A2A task-history storage.

Root cause

The task status update path deep-copies stored history with gob. Interface-held zero-length JSON arrays become typed nil slices during that copy, so A2A JSON changes [] to null.

Fix

Protect empty JSON arrays at the shared A2A executor seam after MCP conversion and before task-history persistence. Explicit nulls and non-empty arrays remain unchanged.

Validation

• Focused ADK A2A, MCP, task-store, and compatibility packages pass.
• Regression tests cover nested empty arrays, non-empty arrays, repeated gob copies, and JSON round trips.
• An isolated Kind/Garden run confirmed raw MCP [] values remain arrays in untouched A2A tasks/get history.

This targets the release/v0.9.x line and adds no dependency or schema change.

[Copilot]

Pull request overview

This PR fixes a subtle JSON fidelity issue in the Go ADK ↔ A2A execution path where structured MCP tool outputs containing non-nil empty arrays ([]) could be transformed into null after A2A task-history persistence (due to gob-based deep copies). It introduces a small protective conversion at the A2A executor boundary and adds regression tests to ensure nested empty arrays remain arrays end-to-end.

Changes:

• Add a gob-stable emptyJSONArray sentinel plus recursive rewriting logic to preserve zero-length JSON arrays during A2A task-history gob copies.
• Apply the preservation step in the A2A executor right after GenAI→A2A part conversion and before task-history persistence.
• Add focused regression tests covering nested empty arrays, repeated gob copies, and JSON/compatibility round trips.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
go/adk/pkg/mcp/structured_content_test.go	Adds an integration-style test asserting nested empty arrays remain non-nil empty arrays in MCP function response events.
go/adk/pkg/a2a/executor.go	Applies preserveEmptyJSONArrays to A2A DataParts before they’re queued/persisted.
go/adk/pkg/a2a/converter.go	Introduces emptyJSONArray (gob-registered) and recursive preservation helpers to keep [] from turning into null.
go/adk/pkg/a2a/converter_test.go	Adds regression coverage for gob deep-copy behavior, JSON round trips, and compatibility conversions.

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