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/resource_template.rb

@@ -2,15 +2,16 @@
 
 module MCP
   class ResourceTemplate
-    attr_reader :uri_template, :name, :title, :description, :icons, :mime_type
+    attr_reader :uri_template, :name, :title, :description, :icons, :mime_type, :meta
 
-    def initialize(uri_template:, name:, title: nil, description: nil, icons: [], mime_type: nil)
+    def initialize(uri_template:, name:, title: nil, description: nil, icons: [], mime_type: nil, meta: nil)
       @uri_template = uri_template
       @name = name
       @title = title
       @description = description
       @icons = icons
       @mime_type = mime_type
+      @meta = meta
     end
 
     def to_h
@@ -21,6 +22,7 @@ module MCP
         description: description,
         icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         mimeType: mime_type,
+        _meta: meta,
       }.compact
     end
   end