feat: test timeout diagnostics with stack traces and deadlock detection

[thomhurst]

Description

TimeoutAttribute exists but provides no diagnostics when a test times out — no stack traces, no deadlock detection, no thread state reporting.

Why

Timeout errors are extremely hard to debug. Users can't tell if it's a real hang vs. a slow test, and have no way to diagnose deadlocks in async code.

Proposed Approach

• Capture stack trace at the moment of timeout
• Report async state (task chains, waiting handles)
• Report thread pool state and lock contention info
• Add --timeout-diagnostics mode for detailed capture
• Generate timeout diagnostics report (HTML/JSON)