fix(ci): resolve dev lint + typecheck failures (CAR-1330)

Three CI-blocking issues on dev branch (also present on uat, fixed in 2b20946):

1. tests/conftest.py — remove extra blank line (ruff format).
2. src/cartsnitch_api/middleware/rate_limit.py — delete duplicate
   _public_limiter/_auth_limiter/_auth_strict_limiter forward-decl block
   (the second occurrence; mypy no-redef).
3. src/cartsnitch_api/cache.py:38 — annotate
   value: str | bytes | None so mypy doesn't widen redis client return
   to Any (no-any-return).

Verified: ruff check . && ruff format --check . && mypy src/cartsnitch_api
all pass.

Sibling of CAR-1330 (which fixes uat directly). Heals dev so future
dev → uat promotions stay green.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/cache.py

@@ -35,7 +35,7 @@ class CacheClient:
     async def get(self, key: str) -> str | None:
         if not self._client:
             return None
-        value = await self._client.get(key)
+        value: str | bytes | None = await self._client.get(key)
         if value is None:
             return None
         if isinstance(value, bytes):