Return void from McpStatelessSyncServer#closeGracefully instead of Mono (#1019)

Co-authored-by: Dariusz Jędrzejczyk <2554306+chemicL@users.noreply.github.com>
Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

MIGRATION-2.0.md

@@ -9,6 +9,7 @@ The changes fall into these areas:
 - [JSON serialization behaviour](#json-serialization-behaviour) — wire-format changes.
 - [Server-side validation](#server-side-validation) — runtime validation of tool arguments and embedded schemas.
 - [Transport changes](#transport-changes) — removed methods and the SSE deprecation.
+- [Server API changes](#server-api-changes) — sync server method signature corrections.
 - [New features](#new-features) — additive, backward-compatible capabilities.
 
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@@ -220,6 +221,18 @@ The HTTP+SSE client and server transports (and their supporting validator/except
 
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+## Server API changes
+
+### `McpStatelessSyncServer#closeGracefully` returns `void`
+
+In 1.x, `McpStatelessSyncServer.closeGracefully()` accidentally leaked the reactive signature from the underlying async server and returned `Mono<Void>`. The sync API is intentionally blocking, so returning a `Mono` was an oversight — callers had to call `.block()` themselves to get any actual shutdown behaviour.
+
+In 2.0 the return type is corrected to `void`; the blocking call is performed internally.
+
+**Action:** Remove any `.block()` (or `.subscribe()`) call you had appended to `closeGracefully()`. The method now blocks until the server has shut down and returns normally.
+
+---
+
 ## New features
 
 These are additive and backward-compatible.