Increase test coverage to 98%: add comprehensive unit tests for client/device, retries, and export paths

## Summary
- Raises repository test coverage from ~81% to 98%
- Adds 45+ targeted unit tests across `fmd_api.client` and `fmd_api.device`
- Focuses on previously untested error paths, retry/backoff logic, and export edge cases
- No production code changes; tests only

## What changed
- New tests in `tests/unit/test_coverage_improvements.py`:
  - Authentication flow: salt retrieval, password hashing, access token, private key decryption
  - Retry/backoff: 429 with Retry-After, 500/502/503/504, connection errors, jitter/no-jitter backoff
  - JSON parsing fallbacks: non-JSON responses, empty body, missing keys
  - Export ZIP: PNG/JPEG detection, default jpg fallback for unknown formats, decrypt errors
  - Device edge cases: photo download decode errors, internal guards
  - Utilities: token masking, Retry-After parsing, streaming responses
- Coverage artifacts: `coverage.xml` generated for Codecov

## Why
- Address Codecov alerts and improve confidence in critical paths (auth, I/O, error handling)
- Reduce regressions by covering negative and edge scenarios

## Metrics
- Before: ~81% total coverage (`client.py` ~77%, `device.py` ~94%)
- After: 98% total coverage (`client.py` ~97%, `device.py` 100%)
- Tests: 104 tests passing locally

## Compatibility
- Breaking changes: None
- Public API changes: None
- Dependencies: No new runtime dependencies

## How to validate locally (optional)
```powershell
# From repo root (Windows PowerShell)
python -m pytest tests/unit --cov=fmd_api --cov-report=term-missing --cov-branch
```

## Notes and follow-ups
- Remaining uncovered lines are hard-to-trigger branches in `client.py` (e.g., specific fallback/log-only segments): `353–358`, `368`, `392–393`, `477–478`, `563`, `717–718`
- Reaching 100% would likely require deeper `aiohttp` response mocking or small refactors to expose those branches for deterministic testing; can be tackled in a follow-up if desired

## Checklist
- [x] All unit tests pass locally
- [x] Coverage ≥ 95% (actual: 98%)
- [x] No production code changes
- [x] `coverage.xml` updated for CI/Codecov

Author: devinslick

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "fmd_api"
-version = "2.0.1"
+version = "2.0.2"
 authors = [{name = "devinslick"}]
 description = "A Python client for the FMD (Find My Device) server API"
 readme = "README.md"