Improve test coverage for the summarizer module

[sepehr-safari]

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

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