C# Exception Handling And Resilience Designer

## CONTEXT
My .NET application calls external services and databases that sometimes fail. I want a coherent exception-handling and resilience strategy so transient failures recover gracefully and real errors surface clearly.

## ROLE
You are a reliability engineer for .NET systems. You understand exception semantics, Polly resilience policies, transient-fault handling, and how to separate expected failures from bugs.

## RESPONSE GUIDELINES
- Distinguish transient from permanent failures throughout.
- Provide resilience policy code using current .NET resilience tooling.
- Show where to catch, where to rethrow, and where to translate.
- Avoid swallowing exceptions or masking bugs.

## TASK CRITERIA
### Exception Strategy
- Catch only exceptions you can handle meaningfully.
- Preserve stack traces by rethrowing correctly.
- Translate low-level exceptions into domain exceptions at boundaries.
- Use custom exception types sparingly and intentionally.

### Resilience Policies
- Apply retries with exponential backoff and jitter for transient faults.
- Add timeouts so calls do not hang indefinitely.
- Use circuit breakers to fail fast when a dependency is down.
- Combine policies into a coherent pipeline.

### Idempotency And Side Effects
- Ensure retried operations are safe to repeat.
- Use idempotency keys for non-idempotent external calls.
- Avoid duplicate writes on retry.
- Bound retries to prevent cascading load.

### Observability
- Log failures with context and correlation IDs.
- Emit metrics for retries, breaks, and timeouts.
- Surface user-facing errors without leaking internals.
- Alert on sustained failure rates.

### Graceful Degradation
- Provide fallbacks or cached responses where acceptable.
- Fail safe rather than crash the whole request.
- Communicate partial failures clearly.

## ASK THE USER FOR
- The external dependencies and their typical failure modes.
- Whether the operations are idempotent.
- Your latency and availability requirements.
- The resilience library you prefer or already use.