fix(mcp): preserve client lifecycle across toolkit copies

[richuV]

Closes #2139.

Summary

• share reference-counted MCP transport ownership across Toolkit copies
• expose Toolkit MCP client inventory and idempotent close
• close final ReActAgent/Harness toolkit resources and roll back failed registration/build
• verify last-owner close, per-copy removal, and failed initialization cleanup

Verification

• mvn -pl agentscope-core -Dtest=McpClientManagerTest test (15/15)
• mvn -pl agentscope-core,agentscope-harness -am -Dtest=ToolkitTest,McpClientManagerTest,ReActAgentTest,HarnessAgentTest,HarnessAgentToolsConfigTest test -Dsurefire.failIfNoSpecifiedTests=false (37/37)
• JCode real stdio MCP lifecycle acceptance (12/12): final Toolkit remove and Harness close terminate the fixture process

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[codecov]

Codecov Report

❌ Patch coverage is 68.80734% with 34 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...e/src/main/java/io/agentscope/core/ReActAgent.java	38.46%	11 Missing and 5 partials ⚠️
.../agentscope/core/tool/McpClientLifecycleOwner.java	73.33%	5 Missing and 3 partials ⚠️
...java/io/agentscope/core/tool/McpClientManager.java	80.00%	3 Missing and 4 partials ⚠️
...src/main/java/io/agentscope/core/tool/Toolkit.java	81.25%	3 Missing ⚠️

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[AgentScopeJavaBot]

🤖 AI Review

This PR resolves #2139 by introducing McpClientLifecycleOwner with reference counting, allowing multiple Toolkit copies to share MCP transport ownership. Key changes: ReActAgent.close() now properly closes the toolkit (was previously a no-op), HarnessAgent.close() propagates delegate.close(), and the build() path adds try-catch/finally for resource rollback on failure. The design is clean and effectively solves the MCP client resource leak issue.

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