Add nx and xx flags to save() for conditional persistence

[abrookins]

Adds support for Redis NX (only set if not exists) and XX (only set if exists) semantics to the save() method.

Usage

# Insert-only: save only if key doesn't exist
result = await model.save(nx=True)
if result is None:
    print("Key already exists, nothing saved")

# Update-only: save only if key already exists  
result = await model.save(xx=True)
if result is None:
    print("Key doesn't exist, nothing saved")

Implementation

• JsonModel: Uses JSON.SET's native NX/XX support (atomic)
• HashModel: Uses EXISTS check before HSET (best-effort, not atomic)
• Returns None if condition not met, the model instance otherwise
• Raises ValueError if both nx and xx are True

Rationale

These flags are standard Redis semantics for conditional writes. Exposing them via save() follows the same pattern as the existing pipeline parameter - giving power users access to Redis performance features when needed.

Closes #703