Bugfix: Sync prompt content to store when switching tabs in agent page

[xuyaqist]

1. 修复 agent 编辑页面切换 tab 时提示词内容未同步到 store 的问题，导致用户编辑agent信息后切换tab导致内容不保存的问题
2. 修复A2A Server返回消息内容不完整的问题

[Copilot]

Pull request overview

This PR addresses two user-facing issues in Nexent: (1) prompt edits on the agent edit page were not reliably persisted when switching tabs, and (2) A2A server responses could lose event fidelity / completeness by collapsing streaming output into plain text.

Changes:

• Frontend: persist the current prompt tab’s form value into the agent config store before switching tabs.
• Backend: collect and return raw SSE event objects (as application/json parts), while persisting only the final_answer text for task/message storage.
• Tests: add unit tests covering stream event collection, part building, event coalescing, and final answer extraction.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
frontend/app/[locale]/agents/components/agentInfo/AgentGenerateDetail.tsx	Saves prompt tab content to the store during tab switches to prevent losing edits.
backend/services/a2a_server_service.py	Preserves raw streaming events and returns them as JSON parts; extracts final_answer for persistence.
test/backend/services/test_a2a_server_service_message.py	Adds tests for new streaming/event helper methods and updated behavior expectations.

Suppressed comments (1)

backend/services/a2a_server_service.py:869

• The terminal taskArtifact event sets lastChunk at the top level but leaves the nested artifact without lastChunk, unlike the adapter’s taskProgress format. Add artifact.lastChunk so consumers can consistently detect completion.

                data={
                    "artifact": {"parts": []},
                    "append": True,
                    "lastChunk": True,
                },

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[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/services/a2a_server_service.py	94.82%	1 Missing and 2 partials ⚠️

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