fix(rmcp): add Audio variant to PromptMessageContent (#865)

The spec's prompt-message ContentBlock union is
`text | image | audio | resource_link | resource`, but
PromptMessageContent omitted `Audio`. Because the enum is
`#[serde(tag = "type")]` with no catch-all, a spec-conformant
`{"type":"audio",...}` content block failed to deserialize with
"unknown variant `audio`", breaking prompts/get for any server that
returns audio prompt content (the audio analogue of #842 / #843).

The supporting AudioContent type already existed, and Audio was
already a variant of the general RawContent enum (tool results,
sampling) -- only PromptMessageContent lacked it.

Add the flattened Audio variant (mirroring Image), a
PromptMessage::new_audio constructor (mirroring new_image), and
serialization + deserialization regression tests.

Fixes #864.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -2115,6 +2115,37 @@
             "mimeType"
           ]
         },
+        {
+          "description": "Audio content with base64-encoded data",
+          "type": "object",
+          "properties": {
+            "annotations": {
+              "anyOf": [
+                {
+                  "$ref": "#/definitions/Annotations"
+                },
+                {
+                  "type": "null"
+                }
+              ]
+            },
+            "data": {
+              "type": "string"
+            },
+            "mimeType": {
+              "type": "string"
+            },
+            "type": {
+              "type": "string",
+              "const": "audio"
+            }
+          },
+          "required": [
+            "type",
+            "data",
+            "mimeType"
+          ]
+        },
         {
           "description": "Embedded server-side resource",
           "type": "object",

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

@@ -2115,6 +2115,37 @@
             "mimeType"
           ]
         },
+        {
+          "description": "Audio content with base64-encoded data",
+          "type": "object",
+          "properties": {
+            "annotations": {
+              "anyOf": [
+                {
+                  "$ref": "#/definitions/Annotations"
+                },
+                {
+                  "type": "null"
+                }
+              ]
+            },
+            "data": {
+              "type": "string"
+            },
+            "mimeType": {
+              "type": "string"
+            },
+            "type": {
+              "type": "string",
+              "const": "audio"
+            }
+          },
+          "required": [
+            "type",
+            "data",
+            "mimeType"
+          ]
+        },
         {
           "description": "Embedded server-side resource",
           "type": "object",