Improve test coverage for the summarizer module #106

Good First Issue

The summarizer module (packages/toolkit/src/core/summarizer.ts) has only 5 tests. Improve coverage by adding edge case tests.

What to add

• Test with empty sessions array
• Test with session having null timestamps
• Test with session having very large duration
• Test with multiple sessions (different transaction IDs)
• Test with session having zero events
• Test summarizeSession() directly with various inputs

Files to modify

• packages/toolkit/src/core/summarizer.test.ts

How to claim

Comment "I'd like to work on this" and it will be assigned to you.

Please hold one open claim at a time. Once the pull request for it is merged,
say which issue you want next and it will be assigned. That keeps issues from
sitting claimed while another is still in review, so other newcomers can see
what is genuinely free.