Change SHOULD to MUST in Lifecycle#Operation

Author: dsp-ant

docs/specification/2025-06-18/basic/lifecycle.mdx

@@ -174,7 +174,7 @@ Capability objects can describe sub-capabilities like:
 During the operation phase, the client and server exchange messages according to the
 negotiated capabilities.
 
-Both parties **SHOULD**:
+Both parties **MUST**:
 
 - Respect the negotiated protocol version
 - Only use capabilities that were successfully negotiated

docs/specification/2025-06-18/changelog.mdx

@@ -28,6 +28,7 @@ the previous revision, [2025-03-26](/specification/2025-03-26).
    tool call results. (PR [#603](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/603))
 8. Require [negotiated protocol version to be specified](/specification/2025-06-18/basic/transports#protocol-version-header)
    via `MCP-Protocol-Version` header in subsequent requests when using HTTP (PR [#548](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/548)).
+9. Change *SHOULD* to *MUST* in [Lifecycle Operation](/specification/2025-06-18/basic/lifecycle#operation)
 
 ## Other schema changes