Replace time.Sleep in tests with deterministic synchronization

[dgageot]

Use require.Eventually, time.AfterFunc, and channel synchronization instead of fixed time.Sleep calls to make tests faster and less flaky.

• telemetry: poll mockHTTP.GetRequestCount() with require.Eventually
• fake/proxy: use channel write and require.Eventually for sync points
• runtime/fallback: use time.AfterFunc for context cancel, require.Eventually for cooldown expiry
• tui/styles: poll atomic callback counter with require.Eventually

Assisted-By: cagent

[docker-agent]

Review Summary

✅ No issues found!

The changes look good. The PR successfully replaces fixed time.Sleep calls with deterministic synchronization patterns:

• ✅ pkg/fake/proxy_test.go: Channel write + require.Eventually for proper sync
• ✅ pkg/runtime/fallback_test.go: time.AfterFunc + require.Eventually for clean timing control
• ✅ pkg/runtime/runtime_test.go: Improved comment explaining timing-based negative test
• ✅ pkg/telemetry/telemetry_test.go: All time.Sleep replaced with require.Eventually polling
• ✅ pkg/tui/styles/theme_watcher_test.go: Proper atomic polling with require.Eventually

All require.Eventually calls use correct syntax, appropriate timeouts, and proper thread-safe operations. No race conditions or synchronization issues detected.

[rumpl]

Can't we use "testing/synctest"

[dgageot]

@rumpl It's coming in the next batch (where it's really useful)