feat(gcp-vertex): add model list with org policy filtering (#6393)

Signed-off-by: rabi <ramishra@redhat.com>

crates/goose/src/providers/base.rs

@@ -108,6 +108,9 @@ pub struct ProviderMetadata {
     pub model_doc_link: String,
     /// Required configuration keys
     pub config_keys: Vec<ConfigKey>,
+    /// Whether this provider allows entering model names not in the fetched list
+    #[serde(default)]
+    pub allows_unlisted_models: bool,
 }
 
 impl ProviderMetadata {
@@ -138,6 +141,7 @@ impl ProviderMetadata {
                 .collect(),
             model_doc_link: model_doc_link.to_string(),
             config_keys,
+            allows_unlisted_models: false,
         }
     }
 
@@ -158,6 +162,7 @@ impl ProviderMetadata {
             known_models: models,
             model_doc_link: model_doc_link.to_string(),
             config_keys,
+            allows_unlisted_models: false,
         }
     }
 
@@ -170,8 +175,15 @@ impl ProviderMetadata {
             known_models: vec![],
             model_doc_link: "".to_string(),
             config_keys: vec![],
+            allows_unlisted_models: false,
         }
     }
+
+    /// Set allows_unlisted_models flag (builder pattern)
+    pub fn with_unlisted_models(mut self) -> Self {
+        self.allows_unlisted_models = true;
+        self
+    }
 }
 
 /// Configuration key metadata for provider setup