test: add unit tests for ActiveStreamingTool

[nuthalapativarun]

Problem

ActiveStreamingTool in core/src/agents/active_streaming_tool.ts has no unit test coverage.

Solution

Add unit tests covering constructor defaults, constructor with params, and field access.

Testing

• All new tests pass: npm test
• No existing tests broken

Checklist

• I have performed a self-review of my code
• Tests added and passing
• Manual E2E verified (N/A — test-only change)

[nuthalapativarun]

Superseded by #464 which combines all three test files into a single PR.