fix: clarify feature flag auth errors

[marandaneto]

💡 Motivation and Context

Relates to #526.

Local feature flag loading uses both the project API key and the personal/feature-flags API key, but 401 errors always blamed only personal_api_key. This keeps the server-provided error detail and asks users to verify both credentials.

💚 How did you test it?

• uv run pytest posthog/test/test_client.py::TestClient::test_load_feature_flags_unauthorized posthog/test/test_feature_flags.py::TestLocalEvaluation::test_load_feature_flags_wrong_key -q
• uv run ruff format --check posthog/client.py posthog/test/test_client.py posthog/test/test_feature_flags.py
• uv run ruff check posthog/client.py posthog/test/test_client.py posthog/test/test_feature_flags.py

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file
• Added the release label to the PR

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
posthog/client.py:1353-1370
**Duplicated message content (OnceAndOnlyOnce)**

The error message body — `"Error loading feature flags: {e.message}. Please verify both your project_api_key and personal_api_key. More information: https://posthog.com/docs/api/overview"` — is constructed twice: once as a `%s` format string for the log call, and again as an f-string for the re-raised `APIError`. Extracting it into a local variable eliminates the duplication and keeps the two outputs in sync automatically.

```suggestion
                detail = (
                    f"Error loading feature flags: {e.message}. "
                    "Please verify both your project_api_key and personal_api_key. "
                    "More information: https://posthog.com/docs/api/overview"
                )
                self.log.error("[FEATURE FLAGS] %s", detail)
                self.feature_flags = []
                self.group_type_mapping = {}
                self.cohorts = {}

                if self.flag_cache:
                    self.flag_cache.clear()

                if self.debug:
                    raise APIError(status=401, message=detail)
```

_{Reviews (1): Last reviewed commit: "fix: clarify feature flag auth errors" | Re-trigger Greptile}

[greptile-apps]

Duplicated message content (OnceAndOnlyOnce)

The error message body — "Error loading feature flags: {e.message}. Please verify both your project_api_key and personal_api_key. More information: https://posthog.com/docs/api/overview" — is constructed twice: once as a %s format string for the log call, and again as an f-string for the re-raised APIError. Extracting it into a local variable eliminates the duplication and keeps the two outputs in sync automatically.

Suggested change

	detail = (
	f"Error loading feature flags: {e.message}. "
	"Please verify both your project_api_key and personal_api_key. "
	"More information: https://posthog.com/docs/api/overview"
	)
	self.log.error("[FEATURE FLAGS] %s", detail)
	self.feature_flags = []
	self.group_type_mapping = {}
	self.cohorts = {}
	if self.flag_cache:
	self.flag_cache.clear()
	if self.debug:
	raise APIError(status=401, message=detail)

Prompt To Fix With AI

This is a comment left during a code review.
Path: posthog/client.py
Line: 1353-1370

Comment:
**Duplicated message content (OnceAndOnlyOnce)**

The error message body — `"Error loading feature flags: {e.message}. Please verify both your project_api_key and personal_api_key. More information: https://posthog.com/docs/api/overview"` — is constructed twice: once as a `%s` format string for the log call, and again as an f-string for the re-raised `APIError`. Extracting it into a local variable eliminates the duplication and keeps the two outputs in sync automatically.

```suggestion
                detail = (
                    f"Error loading feature flags: {e.message}. "
                    "Please verify both your project_api_key and personal_api_key. "
                    "More information: https://posthog.com/docs/api/overview"
                )
                self.log.error("[FEATURE FLAGS] %s", detail)
                self.feature_flags = []
                self.group_type_mapping = {}
                self.cohorts = {}

                if self.flag_cache:
                    self.flag_cache.clear()

                if self.debug:
                    raise APIError(status=401, message=detail)
```

How can I resolve this? If you propose a fix, please make it concise.

[ioannisj]

LG, minor optimization from greptile makes sense