feat(tool): preserve Event block types in SubAgentTool forwardEvent

[jujn]

Description

Close #980
Enhance SubAgentTool forwardEvent to preserve original ContentBlock types and provide empty block fallback.

Checklist

Please check the following items before code is ready to be reviewed.

• Code has been formatted with mvn spotless:apply
• All tests are passing (mvn test)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (e.g. links, examples, etc.)
• Code is ready for review

[Copilot]

Pull request overview

This PR updates SubAgentTool.forwardEvent to forward sub-agent streaming events without losing the original ContentBlock subtypes (e.g., ToolUseBlock), addressing the type-erasure problem described in #980. It also adds a fallback behavior to emit an empty block when an event has no content.

Changes:

• Updated SubAgentTool.forwardEvent to emit the original Msg content blocks (instead of JSON text) and attach event metadata (subagent_event_type, subagent_event_last, etc.).
• Added an empty TextBlock fallback when an event has no content blocks to forward.
• Updated SubAgentToolTest to assert preserved ContentBlock types and correct metadata population.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
agentscope-core/src/main/java/io/agentscope/core/tool/subagent/SubAgentTool.java	Emit original event content blocks and attach event metadata; add empty-block fallback.
agentscope-core/src/test/java/io/agentscope/core/tool/subagent/SubAgentToolTest.java	Update streaming-forwarding test to validate block-type preservation and metadata.

[codecov]

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...io/agentscope/core/tool/subagent/SubAgentTool.java	58.33%	1 Missing and 4 partials ⚠️

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[LearningGp]