refactor(rotator): randomize key selection and streamline daily stats reset

Randomize API key selection lists in `RotatingClient` to ensure a fair distribution of keys, especially when multiple keys have similar usage statistics. This prevents deterministic selection biases and promotes better load balancing.

Consolidate the daily usage statistics reset logic into a dedicated `_reset_daily_stats_if_needed` method within the `UsageManager`. This centralizes and clarifies the reset mechanism, removing inline and potentially inconsistent checks.

src/rotator_library/client.py

@@ -291,7 +291,13 @@ class RotatingClient:
 
         # Establish a global deadline for the entire request lifecycle.
         deadline = time.time() + self.global_timeout
-        keys_for_provider = self.api_keys[provider]
+        
+        # Create a mutable copy of the keys and shuffle it to ensure
+        # that the key selection is randomized, which is crucial when
+        # multiple keys have the same usage stats.
+        keys_for_provider = list(self.api_keys[provider])
+        random.shuffle(keys_for_provider)
+        
         tried_keys = set()
         last_exception = None
         kwargs = self._convert_model_params(**kwargs)
@@ -439,7 +445,11 @@ class RotatingClient:
         """A dedicated generator for retrying streaming completions with full request preparation and per-key retries."""
         model = kwargs.get("model")
         provider = model.split('/')[0]
-        keys_for_provider = self.api_keys[provider]
+        
+        # Create a mutable copy of the keys and shuffle it.
+        keys_for_provider = list(self.api_keys[provider])
+        random.shuffle(keys_for_provider)
+        
         deadline = time.time() + self.global_timeout
         tried_keys = set()
         last_exception = None
@@ -465,7 +475,7 @@ class RotatingClient:
 
                     lib_logger.info(f"Acquiring key for model {model}. Tried keys: {len(tried_keys)}/{len(keys_for_provider)}")
                     current_key = await self.usage_manager.acquire_key(
-                        available_keys=keys_to_try, 
+                        available_keys=keys_to_try,
                         model=model,
                         deadline=deadline
                     )

src/rotator_library/usage_manager.py

@@ -134,6 +134,7 @@ class UsageManager:
         respecting a global deadline.
         """
         await self._lazy_init()
+        await self._reset_daily_stats_if_needed()
         self._initialize_key_states(available_keys)
 
         # This loop continues as long as the global deadline has not been met.
@@ -238,10 +239,6 @@ class UsageManager:
             today_utc_str = datetime.now(timezone.utc).date().isoformat()
             key_data = self._usage_data.setdefault(key, {"daily": {"date": today_utc_str, "models": {}}, "global": {"models": {}}, "model_cooldowns": {}, "failures": {}})
             
-            # Perform a just-in-time daily reset if the date has changed.
-            if key_data["daily"].get("date") != today_utc_str:
-                key_data["daily"] = {"date": today_utc_str, "models": {}}
-
             # Always record a success and reset failures
             model_failures = key_data.setdefault("failures", {}).setdefault(model, {})
             model_failures["consecutive_failures"] = 0