feat(a2a): expose fine-grained agent event stream

[mikemikimike]

Summary

• Add AgentRunner.streamEvents(...) to expose fine-grained AgentEvent streaming for A2A runners.
• Reuse the existing cached-agent lifecycle for legacy and fine-grained streams, including cleanup on completion and error.
• Keep the existing stream(...) API for backward compatibility and add regression coverage.

Closes #2552.

Tests

• mvn -pl agentscope-extensions/agentscope-extensions-protocol/agentscope-extensions-a2a/agentscope-extensions-a2a-server -am clean verify
• mvn -pl agentscope-extensions/agentscope-extensions-protocol/agentscope-extensions-a2a/agentscope-extensions-a2a-server -am -Dtest=AgentRunnerTest,AgentScopeAgentExecutorTest -Dsurefire.failIfNoSpecifiedTests=false test
• git diff --check

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 52.68817% with 44 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...e/a2a/server/executor/AgentScopeAgentExecutor.java	48.83%	35 Missing and 9 partials ⚠️

📢 Thoughts on this report? Let us know!

[zcw888]

PR #2577 added the AgentRunner.streamEvents() capability (fine-grained Flux<AgentEvent>) and implemented it in BaseReActAgentRunner, but the fix looks incomplete: the actual A2A server call chain still uses the legacy coarse-grained stream.

In AgentScopeAgentExecutor.execute() (agentscope-extensions-a2a-server), line 101 still calls agentRunner.stream(...) returning Flux<Event>, and the whole handler chain (BaseFluxEventHandler, BlockingFluxEventHandler, StreamingFluxEventHandler) consumes Event. AgentRunner.streamEvents() is only exercised by AgentRunnerTest and is never consumed by any executor, so A2A clients still cannot receive fine-grained AgentEvents — this is exactly what #2552 asks for.

To fully close this issue, AgentScopeAgentExecutor (and its FluxEventHandler implementations / tests) should be switched to consume streamEvents() / AgentEvent, ideally with a backward-compatible adapter so the existing Event consumers keep working. Could a follow-up PR cover the executor-side wiring?

[oss-maintainer]

Summary

This PR modifies 3 files with 254 lines of diff.

Observations

• Files changed: 3
• Test coverage: ✅ Tests included
• CLA: ✅ Signed

Suggestions

• Please ensure all public API changes are backward compatible
• Consider adding unit tests for new logic paths

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Automated review by github-manager-bot

[mikemikimike]

Addressed the executor-side wiring noted in the review comment:

• AgentScopeAgentExecutor now prefers streamEvents() and routes fine-grained AgentEvent values through the blocking and streaming handlers.
• Legacy custom runners remain supported through an adapter/fallback to stream().
• Added regression coverage for fine-grained execution and legacy fallback.

Validation at 489f67d:

• mvn -pl agentscope-extensions/agentscope-extensions-protocol/agentscope-extensions-a2a/agentscope-extensions-a2a-server -am clean verify ✅ (core: 2,257 tests; A2A client: 109; A2A server: 138; 8 existing core skips)
• git diff --check ✅

The previous required Ubuntu run failed in the unrelated agentscope-harness module; the focused A2A reactor passed locally.