fix: use Interlocked for _sessionTestCount initialization

[thomhurst]

Summary

• `_sessionTestCount` was initialized with direct assignment (`_sessionTestCount = contexts.Count`) but decremented with `Interlocked.Decrement`
• This is a data race: the non-atomic write could be torn or not visible to other threads performing the decrement
• Fix: use `Interlocked.Exchange` for initialization to ensure atomic write with proper memory ordering
• Prevents potential issues with "last test in session" events firing incorrectly

Test plan

• Verify session-level events (first/last test) fire correctly
• Run full engine test suite

🤖 Generated with Claude Code

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

This PR correctly addresses a thread-safety issue by using Interlocked.Exchange for initialization, which properly pairs with the existing Interlocked.Decrement operations on the _sessionTestCount field. The change ensures atomic writes with proper memory barriers, guaranteeing cross-thread visibility. The added documentation comment clearly explains the threading concerns. This is a focused, correct fix that adheres to all project guidelines.