fix(harness): support asynchronous memory flush

[guslegend0510]

Summary

• Add an opt-in MemoryConfig.asyncFlush(boolean) option for HarnessAgent
• Preserve the existing synchronous behavior by default
• Capture a snapshot of completed-call messages before starting the background flush
• Run asynchronous flushes on a bounded scheduler to prevent an unbounded backlog
• Isolate background flush failures from completed agent responses
• Add English and Chinese documentation for the new option

Motivation

HarnessAgent currently waits for the per-call memory flush to complete before completing the response stream.

When the memory model responds slowly, this can significantly delay the agent response. The deprecated ReActAgent.longTermMemoryAsyncRecord(...) option does not apply to HarnessAgent.

This change provides an equivalent opt-in asynchronous mode:

MemoryConfig.builder()
    .asyncFlush(true)
    .build()

The default remains false, preserving the existing completion and persistence semantics.

Behavior

When asyncFlush is enabled:

• The agent response completes without waiting for the memory model
• Messages are copied when the response completes, preventing later calls from changing the flush input
• Failures are logged without failing the completed response
• A dedicated scheduler runs one flush at a time and queues up to three additional tasks
• Saturated tasks are rejected and logged instead of creating an unbounded backlog

Asynchronous flushes are fire-and-forget. In-flight flushes are not awaited during HarnessAgent.close(), so applications that exit immediately may stop before persistence completes.

Testing

mvn -pl agentscope-harness -am test

Results:

• Core: 2294 tests, 0 failures, 9 skipped
• Harness: 839 tests, 0 failures, 3 skipped
• Spotless passed

Fixes #2821

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 15 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...arness/agent/middleware/MemoryFlushMiddleware.java	62.50%	13 Missing and 2 partials ⚠️

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