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/prompt/result_test.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Prompt
+    class ResultTest < ActiveSupport::TestCase
+      test "#to_h returns description and messages" do
+        result = Prompt::Result.new(
+          description: "a prompt",
+          messages: [Prompt::Message.new(role: "user", content: Content::Text.new("hi"))],
+        )
+
+        hash = result.to_h
+
+        assert_equal "a prompt", hash[:description]
+        assert_equal 1, hash[:messages].size
+      end
+
+      test "#to_h includes _meta when present" do
+        meta = { "application/vnd.ant.mcp-app" => { "csp" => "default-src 'self'" } }
+        result = Prompt::Result.new(
+          description: "a prompt",
+          messages: [Prompt::Message.new(role: "user", content: Content::Text.new("hi"))],
+          meta: meta,
+        )
+
+        assert_equal meta, result.to_h[:_meta]
+      end
+
+      test "#to_h omits _meta when nil" do
+        result = Prompt::Result.new(
+          description: "a prompt",
+          messages: [Prompt::Message.new(role: "user", content: Content::Text.new("hi"))],
+        )
+
+        refute result.to_h.key?(:_meta)
+      end
+    end
+  end
+end