openai: Don't send prompt_cache_key for OpenAI-compatible models (#36231)

Some APIs fail when they get this parameter

Closes #36215

Release Notes:

- Fixed OpenAI-compatible providers that don't support prompt caching
and/or reasoning

crates/x_ai/src/x_ai.rs

@@ -105,6 +105,10 @@ impl Model {
         }
     }
 
+    pub fn supports_prompt_cache_key(&self) -> bool {
+        false
+    }
+
     pub fn supports_tool(&self) -> bool {
         match self {
             Self::Grok2Vision