feat(a2a): migrate server runner to streamEvents

[guslegend0510]

Summary

• migrate the A2A server AgentRunner contract from deprecated stream() / Event to streamEvents() / AgentEvent
• update BaseReActAgentRunner to call ReActAgent.streamEvents()
• adapt AgentScopeAgentExecutor to convert typed agent events into A2A streaming artifacts and final messages
• update A2A server tests for typed events, including lifecycle-event filtering

Fixes #2207

Test

• mvn -pl agentscope-extensions/agentscope-extensions-protocol/agentscope-extensions-a2a/agentscope-extensions-a2a-server -am test
• mvn -pl agentscope-extensions/agentscope-spring-boot-starters/agentscope-a2a-spring-boot-starter -am -DskipTests test
• mvn -pl agentscope-extensions/agentscope-spring-boot-starters/agentscope-a2a-spring-boot-starter -am -Dtest=AgentscopeA2aAutoConfigurationTest,A2aJsonRpcControllerTest -DfailIfNoTests=false test

[AgentScopeJavaBot]

🤖 AI Review

This PR migrates the A2A server's AgentRunner contract and AgentScopeAgentExecutor from the deprecated coarse-grained Event / EventType API to the fine-grained typed AgentEvent / AgentEventType hierarchy. The migration is well-aligned with the project's deprecation direction and fixes #2207. However, a critical logic gap in finalMessage population was faithfully carried over from the old code: when requireInnerMessages=false, the finalMessage is never populated because AgentResultEvent handling is gated behind both isLifecycleEvent() (which returns false for AGENT_RESULT) and the requireInnerMessages flag in handleInnerMessage(). This leads to completeTaskWithMessage() being called with a null finalMessage on stream completion, likely causing an NPE.

(inline comments could not be attached — line numbers fell outside PR hunks. See archived report.)

[AgentScopeJavaBot]

🤖 AI Review

This PR migrates the A2A server's AgentRunner contract and AgentScopeAgentExecutor from the deprecated coarse-grained Event / EventType API to the fine-grained typed AgentEvent / AgentEventType hierarchy. The migration is well-aligned with the project's deprecation direction and fixes #2207. However, a critical logic gap in finalMessage population was faithfully carried over from the old code: when requireInnerMessages=false, the finalMessage is never populated because AgentResultEvent handling is gated behind both isLifecycleEvent() (which returns false for AGENT_RESULT) and the requireInnerMessages flag in handleInnerMessage(). This leads to completeTaskWithMessage() being called with a null finalMessage on stream completion, likely causing an NPE.

(inline comments could not be attached — line numbers fell outside PR hunks. See archived report.)

[codecov]

Codecov Report

❌ Patch coverage is 62.79070% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...e/a2a/server/executor/AgentScopeAgentExecutor.java	61.90%	14 Missing and 2 partials ⚠️

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