Spec conformance: meta support and spec updates (#415)

* feat: add _meta to content blocks and embedded resources; update schemas

* feat: set default protocol version; add _meta to content blocks/resources; update schemas

* chore: format content.rs via rustfmt

* chore(protocol): keep LATEST at 2025-03-26 per review until full 2025-06-18 compliance

* feat(prompt): add constructors with optional meta for image and resource

- Keep text helper; meta is currently ignored for text until schema supports it.

* refactor(prompt): simplify constructors so meta is optional; remove duplicate non-meta variants

* fix: modify code comment about version

* refactor(prompt): rename meta parameters in new_resource function for clarity

crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

@@ -176,19 +176,19 @@
             "$ref": "#/definitions/Role"
           }
         },
+        "lastModified": {
+          "type": [
+            "string",
+            "null"
+          ],
+          "format": "date-time"
+        },
         "priority": {
           "type": [
             "number",
             "null"
           ],
           "format": "float"
-        },
-        "timestamp": {
-          "type": [
-            "string",
-            "null"
-          ],
-          "format": "date-time"
         }
       }
     },
@@ -859,6 +859,7 @@
       "type": "object",
       "properties": {
         "_meta": {
+          "description": "Optional protocol-level metadata for this content block",
           "type": [
             "object",
             "null"
@@ -876,6 +877,14 @@
     "RawImageContent": {
       "type": "object",
       "properties": {
+        "_meta": {
+          "description": "Optional protocol-level metadata for this content block",
+          "type": [
+            "object",
+            "null"
+          ],
+          "additionalProperties": true
+        },
         "data": {
           "description": "The base64-encoded image",
           "type": "string"
@@ -933,6 +942,14 @@
     "RawTextContent": {
       "type": "object",
       "properties": {
+        "_meta": {
+          "description": "Optional protocol-level metadata for this content block",
+          "type": [
+            "object",
+            "null"
+          ],
+          "additionalProperties": true
+        },
         "text": {
           "type": "string"
         }

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -168,6 +168,7 @@
       "type": "object",
       "properties": {
         "_meta": {
+          "description": "Optional protocol-level metadata for this content block",
           "type": [
             "object",
             "null"
@@ -292,19 +293,19 @@
             "$ref": "#/definitions/Role"
           }
         },
+        "lastModified": {
+          "type": [
+            "string",
+            "null"
+          ],
+          "format": "date-time"
+        },
         "priority": {
           "type": [
             "number",
             "null"
           ],
           "format": "float"
-        },
-        "timestamp": {
-          "type": [
-            "string",
-            "null"
-          ],
-          "format": "date-time"
         }
       }
     },
@@ -1232,6 +1233,14 @@
           "description": "Image content with base64-encoded data",
           "type": "object",
           "properties": {
+            "_meta": {
+              "description": "Optional protocol-level metadata for this content block",
+              "type": [
+                "object",
+                "null"
+              ],
+              "additionalProperties": true
+            },
             "annotations": {
               "anyOf": [
                 {
@@ -1362,6 +1371,7 @@
       "type": "object",
       "properties": {
         "_meta": {
+          "description": "Optional protocol-level metadata for this content block",
           "type": [
             "object",
             "null"
@@ -1379,6 +1389,14 @@
     "RawImageContent": {
       "type": "object",
       "properties": {
+        "_meta": {
+          "description": "Optional protocol-level metadata for this content block",
+          "type": [
+            "object",
+            "null"
+          ],
+          "additionalProperties": true
+        },
         "data": {
           "description": "The base64-encoded image",
           "type": "string"
@@ -1436,6 +1454,14 @@
     "RawTextContent": {
       "type": "object",
       "properties": {
+        "_meta": {
+          "description": "Optional protocol-level metadata for this content block",
+          "type": [
+            "object",
+            "null"
+          ],
+          "additionalProperties": true
+        },
         "text": {
           "type": "string"
         }