Fix incorrect slog.Error usage for 4xx auth failures in gateway_internal.go

Summary

In platform-api/src/internal/handler/gateway_internal.go, the handlers GetSubscriptions, GetSubscriptionPlans, and GetMCPProxy currently use slog.Error for expected client-side authentication failures (4xx responses), which is inconsistent with the established convention.

Expected Behavior

Per the codebase convention, slog.Warn should be used for expected client errors (4xx responses such as missing/invalid API keys, unauthorized access, bad requests). slog.Error should be reserved for unexpected server-side failures (5xx responses).

The new authenticateRequest helper introduced in #1337 already follows this convention correctly.

Affected Handlers

• GetSubscriptions — uses slog.Error for missing API key and authentication failure (4xx)
• GetSubscriptionPlans — uses slog.Error for missing API key and authentication failure (4xx)
• GetMCPProxy — uses slog.Error for missing API key and authentication failure (4xx)

Fix

Change the above slog.Error calls to slog.Warn for the 4xx auth failure branches in those three handlers, consistent with how authenticateRequest handles them.

References

• PR: https://github.com/wso2/api-platform/pull/1337
• Comment: https://github.com/wso2/api-platform/pull/1337#discussion_r2930592461
• Requested by: @dushaniw