Add _meta field to resource, content, and result classes

## Motivation and Context

MCP 2025-11-25 spec defines a `_meta` general field on many types,
but the Ruby SDK only exposed it on `Tool` and `Prompt`.
To attach metadata (e.g. MCP Apps CSP hints on resource contents),
consumers had to subclass existing classes and override `to_h`.

Add a `meta:` keyword argument and `_meta` serialization to:

- `Resource::Contents`, `Resource::TextContents`, `Resource::BlobContents`
- `Resource`, `ResourceTemplate`
- `Content::Text`, `Content::Image`, `Content::Audio`, `Content::EmbeddedResource`
- `Tool::Response` (`CallToolResult`)
- `Prompt::Result` (`GetPromptResult`)

This aligns the Ruby SDK with the Python and TypeScript SDKs, which both expose `_meta` on these types.

Resolves #308.

## How Has This Been Tested?

Added unit tests for each updated class covering two patterns: `_meta` present
(asserts the value is reflected in `to_h[:_meta]`) and `_meta` omitted
(asserts no `:_meta` key in `to_h`, i.e., `.compact` behavior).
All existing tests continue to pass, and `rake rubocop` is clean.

## Breaking Changes

None. All new `meta:` parameters are optional keyword arguments defaulting to `nil`.
The `to_h` output stays backward-compatible because `nil` values are removed by the existing `.compact` call,
so existing callers require no changes.

test/mcp/resource/contents_test.rb

@@ -70,6 +70,78 @@ module MCP
         assert_equal({ uri: "test://binary", blob: "base64data" }, result)
         refute result.key?(:mimeType)
       end
+
+      test "Contents#to_h omits _meta when nil" do
+        contents = Resource::Contents.new(uri: "test://example", mime_type: "text/plain")
+
+        refute contents.to_h.key?(:_meta)
+      end
+
+      test "Contents#to_h includes _meta when present" do
+        meta = { "application/vnd.ant.mcp-app" => { "csp" => "default-src 'self'" } }
+        contents = Resource::Contents.new(uri: "test://example", mime_type: "text/plain", meta: meta)
+
+        assert_equal meta, contents.to_h[:_meta]
+      end
+
+      test "TextContents#to_h includes _meta when present" do
+        meta = { "application/vnd.ant.mcp-app" => { "csp" => "default-src 'self'" } }
+        text_contents = Resource::TextContents.new(
+          uri: "test://text",
+          mime_type: "text/plain",
+          text: "Hello",
+          meta: meta,
+        )
+
+        result = text_contents.to_h
+
+        assert_equal meta, result[:_meta]
+        assert_equal "Hello", result[:text]
+      end
+
+      test "TextContents#to_h omits _meta when nil" do
+        text_contents = Resource::TextContents.new(
+          uri: "test://text",
+          mime_type: "text/plain",
+          text: "Hello",
+        )
+
+        refute text_contents.to_h.key?(:_meta)
+      end
+
+      test "BlobContents#to_h includes _meta when present" do
+        meta = { "application/vnd.ant.mcp-app" => { "csp" => "default-src 'self'" } }
+        blob_contents = Resource::BlobContents.new(
+          uri: "test://binary",
+          mime_type: "image/png",
+          data: "base64data",
+          meta: meta,
+        )
+
+        result = blob_contents.to_h
+
+        assert_equal meta, result[:_meta]
+        assert_equal "base64data", result[:blob]
+      end
+
+      test "BlobContents#to_h omits _meta when nil" do
+        blob_contents = Resource::BlobContents.new(
+          uri: "test://binary",
+          mime_type: "image/png",
+          data: "base64data",
+        )
+
+        refute blob_contents.to_h.key?(:_meta)
+      end
+
+      test "Contents#to_h preserves empty _meta hash" do
+        contents = Resource::Contents.new(uri: "test://example", mime_type: "text/plain", meta: {})
+
+        result = contents.to_h
+
+        assert result.key?(:_meta)
+        assert_equal({}, result[:_meta])
+      end
     end
   end
 end