Remove unnecessary fields from the tool schemas (#29381)

This PR removes two fields from JSON schemas (`$schema` and `title`),
which are not expected by any model provider, but were spuriously
included by our JSON schema library, `schemars`.

These added noise to requests and cost wasted input tokens.

### Old

```json
{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "FetchToolInput",
  "type": "object",
  "required": [
    "url"
  ],
  "properties": {
    "url": {
      "description": "The URL to fetch.",
      "type": "string"
    }
  }
}
```

### New:

```json
{
  "properties": {
    "url": {
      "description": "The URL to fetch.",
      "type": "string"
    }
  },
  "required": [
    "url"
  ],
  "type": "object"
}
```

- N/A

crates/assistant_tool/src/tool_schema.rs

@@ -10,6 +10,11 @@ pub fn adapt_schema_to_format(
     json: &mut Value,
     format: LanguageModelToolSchemaFormat,
 ) -> Result<()> {
+    if let Value::Object(obj) = json {
+        obj.remove("$schema");
+        obj.remove("title");
+    }
+
     match format {
         LanguageModelToolSchemaFormat::JsonSchema => Ok(()),
         LanguageModelToolSchemaFormat::JsonSchemaSubset => adapt_to_json_schema_subset(json),
@@ -30,10 +35,7 @@ fn adapt_to_json_schema_subset(json: &mut Value) -> Result<()> {
             }
         }
 
-        const KEYS_TO_REMOVE: [&str; 2] = ["format", "$schema"];
-        for key in KEYS_TO_REMOVE {
-            obj.remove(key);
-        }
+        obj.remove("format");
 
         if let Some(default) = obj.get("default") {
             let is_null = default.is_null();