Remove Server#notify_progress broadcast API

## Motivation and Context

`Server#notify_progress` broadcasts progress notifications to all connected clients.
Progress notifications are tied to a specific request's `progressToken` and have no
meaningful broadcast use case. Neither the Python SDK nor the TypeScript SDK provides
a server-level progress broadcast capability.

The MCP specification requires that progress notifications only reference tokens
provided in an active request:

> Progress notifications MUST only reference tokens that:
> - Were provided in an active request
> - Are associated with an in-progress operation

Ref: https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/progress

Broadcasting progress to all clients does not comply with this requirement,
since clients that did not originate the request have no matching `progressToken`.

The introduction of `ServerSession` for per-connection state (#275) made
this removal possible: `Progress` and `ServerContext` now use `notification_target:`
(a `ServerSession`) to route notifications to the originating session only.

### Changes

- Removed `Server#notify_progress` entirely. The method is no longer needed as
  `notification_target` in `call_tool_with_args` now passes `session` directly
  (nil when no session is available, in which case `Progress#report` and
  `ServerContext#notify_log_message` are no-ops).
- Removed `ServerContext#notify_progress` as well. Progress notifications should
  only be sent via `server_context.report_progress`, which enforces the correct
  `progressToken` from the originating request.
- Added nil guards to `Progress#report` and `ServerContext#notify_log_message`
  for when no session is available.
- Rewrote progress tests to use `ServerSession#handle` instead of
  `Server#handle`, reflecting that progress notifications are always session-scoped.
- Removed 4 tests that called `server.notify_progress` directly as a public broadcast API.
- Removed `notify_progress` from the README.md notification methods list.
- Removed the "Server-Side: Direct `notify_progress` Usage" section from the README.md.

Progress notifications should be sent via `server_context.report_progress` inside tool handlers,
which automatically scopes them to the originating client session.

## Breaking Changes

Progress notifications are scoped to a specific request via `progressToken` per the MCP specification,
so the broadcast behavior of `Server#notify_progress` was a spec violation.
This is treated as a bug fix and is made without a deprecation period.
This aligns the Ruby SDK with the Python and TypeScript SDKs, neither of which provides
a server-level progress broadcast API.

`Server#notify_progress` and `ServerContext#notify_progress` are no longer available.
Users should use `server_context.report_progress` inside tool handlers instead, which
provides session-scoped delivery with the correct `progressToken`.

This feature was only recently introduced in mcp 0.9.0:
https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.0

So an early release should help limit its impact on adoption.

lib/mcp/progress.rb

@@ -9,6 +9,7 @@ module MCP
 
     def report(progress, total: nil, message: nil)
       return unless @progress_token
+      return unless @notification_target
 
       @notification_target.notify_progress(
         progress_token: @progress_token,

lib/mcp/server.rb

@@ -116,7 +116,7 @@ module MCP
     # @param request [Hash] A parsed JSON-RPC request.
     # @param session [ServerSession, nil] Per-connection session. Passed by
     #   `ServerSession#handle` for session-scoped notification delivery.
-    #   When `nil`, notifications broadcast to all sessions.
+    #   When `nil`, progress and logging notifications from tool handlers are silently skipped.
     # @return [Hash, nil] The JSON-RPC response, or `nil` for notifications.
     def handle(request, session: nil)
       JsonRpcHandler.handle(request) do |method|
@@ -129,7 +129,7 @@ module MCP
     # @param request [String] A JSON-RPC request as a JSON string.
     # @param session [ServerSession, nil] Per-connection session. Passed by
     #   `ServerSession#handle_json` for session-scoped notification delivery.
-    #   When `nil`, notifications broadcast to all sessions.
+    #   When `nil`, progress and logging notifications from tool handlers are silently skipped.
     # @return [String, nil] The JSON-RPC response as JSON, or `nil` for notifications.
     def handle_json(request, session: nil)
       JsonRpcHandler.handle_json(request) do |method|
@@ -186,21 +186,6 @@ module MCP
       report_exception(e, { notification: "resources_list_changed" })
     end
 
-    def notify_progress(progress_token:, progress:, total: nil, message: nil)
-      return unless @transport
-
-      params = {
-        "progressToken" => progress_token,
-        "progress" => progress,
-        "total" => total,
-        "message" => message,
-      }.compact
-
-      @transport.send_notification(Methods::NOTIFICATIONS_PROGRESS, params)
-    rescue => e
-      report_exception(e, notification: "progress")
-    end
-
     def notify_log_message(data:, level:, logger: nil)
       return unless @transport
       return unless logging_message_notification&.should_notify?(level)
@@ -524,9 +509,8 @@ module MCP
       args = arguments&.transform_keys(&:to_sym) || {}
 
       if accepts_server_context?(tool.method(:call))
-        notification_target = session || self
-        progress = Progress.new(notification_target: notification_target, progress_token: progress_token)
-        server_context = ServerContext.new(context, progress: progress, notification_target: notification_target)
+        progress = Progress.new(notification_target: session, progress_token: progress_token)
+        server_context = ServerContext.new(context, progress: progress, notification_target: session)
         tool.call(**args, server_context: server_context).to_h
       else
         tool.call(**args).to_h

lib/mcp/server_context.rb

@@ -18,22 +18,14 @@ module MCP
       @progress.report(progress, total: total, message: message)
     end
 
-    # Sends a progress notification scoped to the originating session.
-    #
-    # @param progress_token [String, Integer] The token identifying the operation.
-    # @param progress [Numeric] Current progress value.
-    # @param total [Numeric, nil] Total expected value.
-    # @param message [String, nil] Human-readable status message.
-    def notify_progress(progress_token:, progress:, total: nil, message: nil)
-      @notification_target.notify_progress(progress_token: progress_token, progress: progress, total: total, message: message)
-    end
-
     # Sends a log message notification scoped to the originating session.
     #
     # @param data [Object] The log data to send.
     # @param level [String] Log level (e.g., `"debug"`, `"info"`, `"error"`).
     # @param logger [String, nil] Logger name.
     def notify_log_message(data:, level:, logger: nil)
+      return unless @notification_target
+
       @notification_target.notify_log_message(data: data, level: level, logger: logger)
     end