Remove undocumented handler override methods

## Motivation and Context

The following public methods allow replacing the SDK's built-in request handlers:

- `tools_call_handler`
- `tools_list_handler`
- `resources_list_handler`
- `resources_templates_list_handler`
- `prompts_list_handler`
- `prompts_get_handler`

These were introduced in Internal Release 0.2.0 (6004f42) as part
of "allow setting handlers on Server", before declarative APIs (`define_tool`, `Prompt` class, `Resource` class) were
fully established.

Now that those declarative APIs exist, these handler overrides are redundant: none are documented in the README.md,
and no usage exists outside of the SDK's own tests.

These handler overrides also create architectural problems. Since custom handlers cannot receive session context,
they would not work correctly with a per-session architecture, such as session-scoped notifications.

### Changes

- Removed `tools_call_handler`, `tools_list_handler`, `resources_list_handler`,
`resources_templates_list_handler`, `prompts_list_handler`, and `prompts_get_handler`
from `Server`.
- Removed the associated tests.
- Retained `resources_read_handler` (documented in the README.md and actively used
in examples and conformance server, because resource reading is application-specific).

## Breaking Change

The following methods are removed without a deprecation period:
`tools_call_handler`, `tools_list_handler`, `resources_list_handler`,
`resources_templates_list_handler`, `prompts_list_handler`,
`prompts_get_handler`.

These were never documented in the README.md and have no known usage
outside of the SDK's own tests. As a result, little to no impact on
users is expected, and a deprecation warning would have no practical
audience. Users who relied on these should use `define_tool`,
prompt/resource registration, or `resources_read_handler` instead.

lib/mcp/server.rb

@@ -199,34 +199,16 @@ module MCP
       report_exception(e, { notification: "log_message" })
     end
 
-    def resources_list_handler(&block)
-      @handlers[Methods::RESOURCES_LIST] = block
-    end
-
+    # Sets a custom handler for `resources/read` requests.
+    # The block receives the parsed request params and should return resource
+    # contents. The return value is set as the `contents` field of the response.
+    #
+    # @yield [params] The request params containing `:uri`.
+    # @yieldreturn [Array<Hash>, Hash] Resource contents.
     def resources_read_handler(&block)
       @handlers[Methods::RESOURCES_READ] = block
     end
 
-    def resources_templates_list_handler(&block)
-      @handlers[Methods::RESOURCES_TEMPLATES_LIST] = block
-    end
-
-    def tools_list_handler(&block)
-      @handlers[Methods::TOOLS_LIST] = block
-    end
-
-    def tools_call_handler(&block)
-      @handlers[Methods::TOOLS_CALL] = block
-    end
-
-    def prompts_list_handler(&block)
-      @handlers[Methods::PROMPTS_LIST] = block
-    end
-
-    def prompts_get_handler(&block)
-      @handlers[Methods::PROMPTS_GET] = block
-    end
-
     private
 
     def validate!