docs: enable encryption examples by setting CACHEKIT_MASTER_KEY in test fixtures

- Add os.environ["CACHEKIT_MASTER_KEY"] = secret_key to conftest.py
- Convert encryption examples in api-reference.md to testable
- 134 doc tests now pass (up from 133)

Author: 27Bslash6

docs/api-reference.md

@@ -658,8 +658,8 @@ def default_function():
     }
 ```
 
-**Encryption examples** (require `CACHEKIT_MASTER_KEY` env var):
-```python notest
+**Encryption examples** (env var `CACHEKIT_MASTER_KEY` set in test fixtures):
+```python
 from cachekit import cache
 from cachekit.serializers import EncryptionWrapper, OrjsonSerializer
 
@@ -669,7 +669,7 @@ def get_user_ssn(user_id: int):
     return {"ssn": "123-45-6789", "dob": "1990-01-01"}
 
 # Encrypted JSON (zero-knowledge API caching)
-@cache(serializer=EncryptionWrapper(serializer=OrjsonSerializer()), backend=None)
+@cache(serializer=EncryptionWrapper(serializer=OrjsonSerializer(), master_key=bytes.fromhex(secret_key)), backend=None)
 def get_api_keys(tenant_id: str):
     return {"api_key": "sk_live_...", "webhook_secret": "whsec_..."}
 ```

docs/conftest.py

@@ -6,6 +6,7 @@
 """
 
 import logging
+import os
 import time
 
 try:
@@ -92,6 +93,8 @@ def expensive_computation():
 
     # Secret key for encryption examples (test value only)
     secret_key = "a" * 64  # 32 bytes in hex
+    # Set env var so @cache.secure validation passes
+    os.environ["CACHEKIT_MASTER_KEY"] = secret_key
 
     globals_dict = {
         "cache": cache,