Add warning log for local flag evaluation cold start (#452)

* Add warning log when local flag evaluation called before flags loaded

When feature_enabled() is called with only_evaluate_locally=True before
flag definitions are fetched, the SDK silently returns None. This adds a
warning log so users can diagnose the issue immediately.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Move cold start warning to only fire for only_evaluate_locally=True

The warning was in _locally_evaluate_flag which runs for all flag
evaluations, including those that fall back to server-side evaluation.
Move it to the caller where only_evaluate_locally is known, so it only
fires when the caller explicitly opted out of the server fallback.

* Narrow cold start warning to only fire when flags were never fetched

Use `is None` instead of `not` to avoid firing when flags are loaded
but empty (401, 402, no personal_api_key), which already have their
own specific error logs.

* add changeset

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: dmarticus

.sampo/changesets/forthright-seer-otso.md

@@ -0,0 +1,5 @@
+---
+pypi/posthog: patch
+---
+
+Add warning log for local flag evaluation cold start

posthog/client.py

@@ -1620,7 +1620,13 @@ def _get_feature_flag_result(
                 self.flag_cache.set_cached_flag(
                     distinct_id, key, flag_result, self.flag_definition_version
                 )
-        elif not only_evaluate_locally:
+        elif only_evaluate_locally:
+            if self.feature_flags is None:
+                self.log.warning(
+                    "[FEATURE FLAGS] Local evaluation called but feature flag definitions are not loaded yet. "
+                    "Returning None. You can call load_feature_flags() to load flags explicitly."
+                )
+        else:
             try:
                 flag_details, request_id, evaluated_at, errors_while_computing = (
                     self._get_feature_flag_details_from_server(