Support class-based Resource and ResourceTemplate definitions

## Motivation and Context

Resolve the design difference raised in issue #98: Tools and Prompts support class-based definitions
while Resources and Resource Templates were plain data objects requiring a `resources_read_handler` block
with manual URI routing.

`MCP::Resource` and `MCP::ResourceTemplate` now support the same class-level DSL as `MCP::Tool`
and `MCP::Prompt` (`uri`, `resource_name`, `title`, `description`, `icons`, `mime_type`, `annotations`,
`size`, `meta`, and `uri_template` / `resource_template_name` for templates), plus `define` factory methods
and `MCP::Server#define_resource` / `#define_resource_template`.

The default `resources/read` handler now routes requests automatically: an exact URI match calls
the class-based resource's `contents` method, and class-based resource templates match
simple RFC 6570 level 1 `{var}` expressions, passing extracted variables as keyword arguments.
`contents` can opt in to a `server_context:` keyword argument. When class-based entries are registered
and no match is found, the server responds with -32602 carrying the URI in error data per SEP-2164.

## How Has This Been Tested?

- Added regression tests covering the class-level DSL, URI template matching, auto-routing,
  `server_context:` opt-in, error responses, and backward compatibility of the no-op default.
- Verified end-to-end with a smoke script exercising exact-match reads, template-match reads,
  unknown URIs, and legacy instance-only setups.

## Breaking Changes

No. Instance-based registration, `resources_read_handler` precedence, and the no-op `[]` response for
setups without class-based entries are unchanged. Reassigning `Server#resources` now rebuilds
the internal URI index.

Closes #98

docs/building-servers.md

@@ -291,20 +291,28 @@ class MyResource < MCP::Resource
   resource_name "config"
   description "Application configuration"
   mime_type "application/json"
+
+  class << self
+    def contents
+      [MCP::Resource::TextContents.new(
+        uri: uri,
+        mime_type: mime_type,
+        text: File.read("config.json")
+      )]
+    end
+  end
 end
 
 server = MCP::Server.new(
   name: "my_server",
-  resources: [MyResource],
-  resources_read_handler: ->(uri, _server_context) {
-    case uri
-    when "file:///data/config.json"
-      { uri: uri, text: File.read("config.json"), mimeType: "application/json" }
-    end
-  }
+  resources: [MyResource]
 )
 ```
 
+The server automatically routes `resources/read` requests to the matching class-based resource's `contents` method.
+Requests for unregistered URIs respond with the JSON-RPC Invalid Params error (`-32602`). To handle reads manually instead,
+register a block with `server.resources_read_handler`, which fully replaces the automatic routing.
+
 ## Configuration
 
 ```ruby