Add integration tests for optional configuration provider resilience

Tests verify three scenarios using secretstores.local.file (no external deps):
- optional:true with delayed sidecar: config populates once sidecar starts
- optional:false (default): Build() blocks until sidecar ready, secrets populated
- Disposal during background load: exits cleanly with no hang

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: George Tsiokos <george@tsiokos.com>

Author: GeorgeTsiokos