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.

lib/mcp/prompt/result.rb

@@ -3,15 +3,16 @@
 module MCP
   class Prompt
     class Result
-      attr_reader :description, :messages
+      attr_reader :description, :messages, :meta
 
-      def initialize(description: nil, messages: [])
+      def initialize(description: nil, messages: [], meta: nil)
         @description = description
         @messages = messages
+        @meta = meta
       end
 
       def to_h
-        { description: description, messages: messages.map(&:to_h) }.compact
+        { description: description, messages: messages.map(&:to_h), _meta: meta }.compact
       end
     end
   end