[Doc] Fix server_context description for Exception Reporting

## Motivation and Context

The README's Exception Reporting section claimed that `server_context` includes
`{ tool_name: "name", arguments: { ... } }` for tool calls. This matched
the implementation in 0.3.0 through 0.5.0, but the `{ tool_name:, arguments: }`
context was removed in 0.5.1 when exception reporting was consolidated into
the centralized `{ request: request }` path. The README was never updated to reflect this.

`Server#report_exception` is now only called with `{ request: request }`
(from `handle_request`) or `{ notification: "..." }` (from notification delivery rescues),
and tool call failures flow through the `{ request: ... }` path after being wrapped in
a `RequestHandlerError`.

The stale description misled users into expecting `tool_name` and `arguments`
in their exception reporter callbacks.

## How Has This Been Tested?

Documentation-only change. No behavioral modification.

## Breaking Changes

None.

README.md

@@ -423,10 +423,10 @@ The exception reporter receives two arguments:
 - `exception`: The Ruby exception object that was raised
 - `server_context`: A hash containing contextual information about where the error occurred
 
-The server_context hash includes:
+The `server_context` hash includes:
 
-- For tool calls: `{ tool_name: "name", arguments: { ... } }`
-- For general request handling: `{ request: { ... } }`
+- For request handling failures: `{ request: { ... } }` (the raw JSON-RPC request hash)
+- For notification delivery failures: `{ notification: "tools_list_changed" }` (or the relevant notification name)
 
 When an exception occurs: