fix: wire PII redaction + token empty warnings for plugin platforms

PII redaction: build_session_context_prompt() now checks the plugin
registry's pii_safe flag in addition to the hardcoded _PII_SAFE_PLATFORMS
frozenset. Plugin platforms that set pii_safe=True (e.g. phone-based
messaging bridges) get their user IDs redacted before LLM context.

Token empty warnings: the empty-token diagnostic at config load now
checks the plugin registry's required_env when a platform isn't in the
hardcoded _token_env_names dict. Catches 'enabled but empty' for
plugin platforms too.

gateway/config.py

@@ -891,6 +891,15 @@ def _validate_gateway_config(config: "GatewayConfig") -> None:
         if not pconfig.enabled:
             continue
         env_name = _token_env_names.get(platform)
+        if not env_name:
+            # Check plugin registry for required_env
+            try:
+                from gateway.platform_registry import platform_registry
+                entry = platform_registry.get(platform.value)
+                if entry and entry.required_env:
+                    env_name = entry.required_env[0]  # primary env var
+            except Exception:
+                pass
         if env_name and pconfig.token is not None and not pconfig.token.strip():
             logger.warning(
                 "%s is enabled but %s is empty. "