PHP: Fix constant refrrences format and add missing objectExists option for PHP templates

[spinillos]

It fixes Constant References format and allows us to use objectExists function in templates.

[github-actions]

Note: the diff show code changes that would be introduced by this PR to the output of the config/foundation_sdk.dev.yaml codegen pipeline (dev version of the Foundation SDK).

🔎 Changes to config/foundation_sdk.dev.yaml

diff --new-file --unidirectional-new-file '--color=never' --unified --recursive '--exclude=.git' '--exclude=gradle.properties' '--exclude=pyproject.toml' '--exclude=package.json' '--exclude=*.md' /tmp/foundation-workspace-main/foundation-sdk/java/src/main/java/com/grafana/foundation/common/BoolOrFloat64Deserializer.java /tmp/foundation-workspace-current/foundation-sdk/java/src/main/java/com/grafana/foundation/common/BoolOrFloat64Deserializer.java
--- /tmp/foundation-workspace-main/foundation-sdk/java/src/main/java/com/grafana/foundation/common/BoolOrFloat64Deserializer.java	2025-10-24 14:29:22.170372825 +0000
+++ /tmp/foundation-workspace-current/foundation-sdk/java/src/main/java/com/grafana/foundation/common/BoolOrFloat64Deserializer.java	2025-10-24 14:28:40.219239103 +0000
@@ -13,6 +13,12 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 
 import java.io.IOException;
+import com.grafana.foundation.cog.variants.Dataquery;
+import com.grafana.foundation.cog.variants.Registry;
+import com.grafana.foundation.cog.variants.PanelConfig;
+import com.grafana.foundation.common.DataSourceRef;
+import java.util.Map;
+import java.util.List;
 
 public class BoolOrFloat64Deserializer extends JsonDeserializer<BoolOrFloat64> {