feat(tool): expose API to add registered tool to group

[guslegend0510]

AgentScope-Java Version

2.0.3-SNAPSHOT

Description

Closes #2835.

ToolGroupManager already supports an n:m relationship between tools and tool groups, but Toolkit did not expose a public API for assigning an already-registered tool to an additional group.

This PR:

• Adds Toolkit#addToolToGroup(String groupName, String toolName).
• Validates that both the target group and tool exist.
• Preserves the existing registered tool instance and metadata.
• Keeps repeated assignments idempotent.
• Replaces the duplicate-registration workaround in the multi-group regression test.
• Adds validation coverage for unknown groups and unregistered tools.

Testing

• mvn -pl agentscope-core -Dtest=ToolkitTest test
  • 37 tests passed
• mvn -pl agentscope-core test
  • 2295 tests passed
  • 0 failures
  • 0 errors
  • 9 skipped
• git diff --check
• Spotless check passed

Checklist

• Code formatting has been verified with Spotless
• All affected module tests are passing
• Javadoc comments are complete and follow project conventions
• Public API behavior is covered by regression tests
• Code is ready for review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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