fix: add Dragonfly compatibility for Lua scripts

[vyavdoshenko]

This PR adds compatibility with Dragonfly - a Redis-compatible in-memory store. Two issues are addressed:

1. Undeclared keys in Lua scripts

Dragonfly requires all keys accessed in Lua scripts to be declared upfront in the KEYS array, or the script must explicitly opt out using the --!df flags=allow-undeclared-keys directive.

Cacheops dynamically generates conjunction keys (conj:*) inside Lua scripts, which causes failures in Dragonfly without this flag.

Related Dragonfly issues:

• Dragonfly does not allow accessing undeclared keys in lua scripts dragonflydb/dragonfly#272
• simplify lua_undeclared_keys_shas management dragonflydb/dragonfly#6119
• chore: more fixes for django-cacheops compatibility dragonflydb/dragonfly#6129

2. cjson integer formatting difference

Dragonfly's cjson.decode converts JSON integers to Lua floats, causing tostring(1) to return "1.0" instead of "1". This broke cache key matching between Python-generated keys and Lua-generated keys:

Python: conj:auth_user:id=1
Lua:    conj:auth_user:id=1.0  ← mismatch!

Added format_val() helper function to format integers without a decimal point.

Changes:

• Added --!df flags=allow-undeclared-keys directive to all Lua scripts
• Added format_val() function to handle integer formatting consistently

Testing:

Dragonfly:

# Start Dragonfly
docker run -d --name dragonfly -p 6379:6379 docker.dragonflydb.io/dragonflydb/dragonfly

# Run tests
pytest

Redis compatibility:
Tested with Redis 4.x and Redis 7.x - all tests pass:

Redis 4:

docker run -d --name redis4 -p 6379:6379 redis:4-alpine
pytest

Redis 7:

docker run -d --name redis7 -p 6379:6379 redis:7-alpine
pytest

Notes:

• The --!df directive is a Lua comment, so it's safely ignored by Redis
• The format_val() fix is defensive and works correctly on both Redis and Dragonfly