[Zhoujie628]

Summary

This PR contains 3 commits addressing three correctness issues in the A2A-T server-side Spring wiring.

1. fix: keep workbench notification subscriptions state-independent

Decouples workbench Notification-T subscriptions from workflow Task-T session state; a subscription stays open until a recovery result, explicit cancellation, or workbench shutdown.

2. fix(a2a): apply configured blocking timeouts to the builder-built request handler

• a2a-java's DefaultRequestHandler.Builder.build() hardcodes blocking-call timeouts (agent=5s / consumption=2s / reconciliation=1s) and never consults A2AConfigProvider (only the CDI @PostConstruct path reads it). Under Spring wiring the configuration is therefore silently ignored: a blocking message:send returns a WORKING snapshot after 5 seconds while the agent keeps running — a silent failure for any real agent task (LLM calls, multi-step workflow steps).
• Fix: apply the A2AProperties values directly after build() via reflection (fails fast if the SDK field layout changes) and log the effective values.
• A2AProperties defaults 30/5/1 → 900/960/60 seconds; positive() validation kept (0 remains rejected; disabling blocking is the caller's returnImmediately choice, not a zero timeout).
• The new test exercises the real build→apply path (reflectively reads back the handler fields, asserting the 5s baseline first) instead of only asserting the ConfigProvider mapping.

3. feat(a2a): echo requested supported extensions via A2A-Extensions response header

Success responses now echo the A2A-Extensions header listing the requested extensions that the AgentCard declares, so extension-aware clients can verify server participation per the A2A extension mechanism. Requested-but-undeclared extensions are not activated and stay out of the list; error responses are unchanged. Covered by MockMvc tests for the supported, unsupported, and no-declaration cases.

Verification

• Full reactor mvn -B test BUILD SUCCESS (workflow-engine, spring-boot-starter, samples)
• spotless:apply formatting; google-java-format check passes
• All three commits carry a DCO Signed-off-by trailer

Follow-up

After merge, publish net.openan.workflow.sdk:spring-boot-starter:0.1.2 to Maven Central (the current Central latest, 0.1.1, still ships the blocking-timeout bug).