⚙️ FEATURE-#287: Expand Storage ABC with delete, list, TTL, CAS

[FernandoCelmer]

Closes #287

Description

• dotflow/abc/storage.py — Storage ABC gains five new abstract methods: delete, delete_prefix, list_keys, atomic_swap; post signature extended with optional ttl and fingerprint; clear demoted to a concrete default that delegates to delete_prefix
• dotflow/providers/storage_default.py — Full in-memory implementation with threading.RLock, TTL via time.monotonic, lazy eviction on get/list_keys
• dotflow/providers/storage_file.py — File-backed implementation with sidecar .meta files for TTL/fingerprint; threading.RLock added (currently only used in atomic_swap)
• dotflow/providers/storage_s3.py — S3 implementation delegating new operations to the S3 service class
• dotflow/providers/storage_gcs.py — GCS implementation delegating new operations to the GCS service class
• dotflow/cloud/aws/services/s3.py — Adds delete and list_keys to the S3 service layer
• dotflow/cloud/gcp/services/gcs.py — Adds delete and list_keys to the GCS service layer
• dotflow/testing/storage_contract.py — New reusable StorageContract mixin for third-party driver authors
• dotflow/testing/__init__.py — Exports StorageContract
• tests/providers/test_storage_default_contract.py — Runs StorageContract against StorageDefault
• tests/providers/test_storage_file_contract.py — Runs StorageContract against StorageFile

Motivation and Context

The existing Storage ABC exposed only post, get, key, and clear, which was insufficient for workflows that need TTL-based expiration, fingerprint-based deduplication, enumeration of stored keys, targeted single-key deletion, and compare-and-swap updates. Closes #287.

Types of changes

• Bug fix (change that fixes an issue)
• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly

[FernandoCelmer]

🔍 Code Review

Code issues found: 5

#	Severity	Comment
1	[Blocking]	S3.delete catches non-existent exception type
2	[Blocking]	atomic_swap inherits stale TTL in StorageDefault
3	[Blocking]	atomic_swap cannot propagate TTL in StorageFile
4	[Blocking]	StorageFile lock only protects atomic_swap
5	[Suggestion]	S3/GCS atomic_swap is not truly atomic

[FernandoCelmer]

Code Review (re-review after fixes)

All 5 issues from previous review have been resolved:

1. S3 delete — now uses from botocore.exceptions import ClientError with proper 404 code filtering
2. StorageDefault.atomic_swap — clears old TTL/fingerprint before writing, applies new TTL if provided
3. StorageFile.atomic_swap — entire swap operation now within single with self._lock: block
4. StorageFile lock coverage — post() and get() now acquire self._lock
5. S3/GCS atomic_swap — now uses conditional writes (S3 IfMatch/IfNoneMatch ETags, GCS if_generation_match). True CAS semantics.

Verdict: All blockers resolved. Ready to merge.