feat(rmcp): add optional _meta to CallToolResult, EmbeddedResource, and ResourceContents (#386)

* feat(rmcp): support optional _meta on EmbeddedResource and ResourceContents

- Add optional _meta to RawEmbeddedResource and ResourceContents (text/blob)
- Update constructors/usages to set meta: None by default
- Add tests to lock behavior (embedded text/blob + CallToolResult)
- Update JSON Schemas to include optional _meta fields

* style(rmcp): rustfmt after _meta changes and tests

crates/rmcp/src/model.rs

@@ -1267,6 +1267,9 @@ pub struct CallToolResult {
     /// Whether this result represents an error condition
     #[serde(skip_serializing_if = "Option::is_none")]
     pub is_error: Option<bool>,
+    /// Optional protocol-level metadata for this result
+    #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+    pub meta: Option<Meta>,
 }
 
 impl CallToolResult {
@@ -1276,6 +1279,7 @@ impl CallToolResult {
             content,
             structured_content: None,
             is_error: Some(false),
+            meta: None,
         }
     }
     /// Create an error tool result with unstructured content
@@ -1284,6 +1288,7 @@ impl CallToolResult {
             content,
             structured_content: None,
             is_error: Some(true),
+            meta: None,
         }
     }
     /// Create a successful tool result with structured content
@@ -1305,6 +1310,7 @@ impl CallToolResult {
             content: vec![Content::text(value.to_string())],
             structured_content: Some(value),
             is_error: Some(false),
+            meta: None,
         }
     }
     /// Create an error tool result with structured content
@@ -1330,6 +1336,7 @@ impl CallToolResult {
             content: vec![Content::text(value.to_string())],
             structured_content: Some(value),
             is_error: Some(true),
+            meta: None,
         }
     }
 
@@ -1377,6 +1384,9 @@ impl<'de> Deserialize<'de> for CallToolResult {
             structured_content: Option<Value>,
             #[serde(skip_serializing_if = "Option::is_none")]
             is_error: Option<bool>,
+            /// Accept `_meta` during deserialization
+            #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+            meta: Option<Meta>,
         }
 
         let helper = CallToolResultHelper::deserialize(deserializer)?;
@@ -1384,6 +1394,7 @@ impl<'de> Deserialize<'de> for CallToolResult {
             content: helper.content.unwrap_or_default(),
             structured_content: helper.structured_content,
             is_error: helper.is_error,
+            meta: helper.meta,
         };
 
         // Validate mutual exclusivity