fix: preserve observer setup failure at readiness timeout

[abrichr]

Fixes the macOS main-branch failure in Tests run 30144317642, job 89643176438.

When readiness timed out while the observer thread was concurrently completing setup with an error, start() always surfaced the generic timeout. The transactional abort now joins first and prefers the concrete first observer failure recorded during that abort. A genuinely never-ready observer still returns the readiness timeout.

Validation:

• 34/34 tests in tests/test_input_observer.py pass locally
• synchronized regression covers timeout-before-setup-failure ordering
• original failed-start and clean never-ready cases pass
• Ruff check passes for openadapt_capture

No GUI or input injection, package metadata, Capture 1.1 contract, or distribution boundary changes.