fix(harness): add explicit lifecycle management for spawned agents

[guslegend0510]

Summary

• add an explicit agent_release tool for retained subagents
• scope live subagent registries by (userId, parentSessionId)
• manage spawned agents with ACTIVE, RELEASING, and RELEASED lifecycle states
• track in-flight invocations across synchronous, asynchronous, cancelled, remote, and timeout-promoted executions
• atomically remove live keys, labels, and persisted spawn registry entries during release
• revoke exposed Gateway handles and close AutoCloseable child agents
• prevent released agents from being restored from stale state
• close factory-created candidates that lose concurrent label or persistent-key races
• make agent_list runtime-context aware
• preserve persistent session state when the parent HarnessAgent is closed
• update the English and Chinese subagent documentation

Completed tasks are intentionally not released automatically. A retained subagent remains addressable for follow-up agent_send calls until agent_release is called.

agent_release returns busy while an invocation is still running. No TTL, LRU, force-release option, or automatic size limit is introduced.

Tests

• added lifecycle regression coverage for 256 consecutive spawn/release cycles
• covered send by key and label before and after release
• covered cross-session isolation and state restoration
• covered synchronous, asynchronous, cancellation, and timeout-promotion leases
• covered concurrent duplicate labels and persistent keys
• covered send/release and restore/release races
• covered Gateway handle revocation and parent-agent cleanup
• verified tests do not depend on Thread.sleep or System.gc

Validation completed:

• focused AgentSpawnTool and middleware tests: 69 passed
• mvn -pl agentscope-harness -am test
• Spotless
• git diff --check
• jupyter-book build .

Fixes #2915

[codecov]

Codecov Report

❌ Patch coverage is 65.76200% with 164 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../agentscope/harness/agent/tool/AgentSpawnTool.java	67.53%	88 Missing and 49 partials ⚠️
.../harness/agent/middleware/SubagentsMiddleware.java	50.00%	10 Missing ⚠️
...s/agent/middleware/DynamicSubagentsMiddleware.java	46.66%	6 Missing and 2 partials ⚠️
...tscope/harness/agent/gateway/GatewayBootstrap.java	0.00%	5 Missing ⚠️
...pe/harness/agent/subagent/task/BackgroundTask.java	62.50%	1 Missing and 2 partials ⚠️
...e/harness/agent/gateway/SubagentGatewayBridge.java	0.00%	1 Missing ⚠️

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