fix: Align ElicitResult action naming with specification

Change action value from "decline" to "reject" to match the MCP specification.

The specification defines three actions for Elicitation:
- accept
- reject
- cancel

But the Python SDK was using "decline" instead of "reject". This fixes
the inconsistency between the SDK and the specification.

Fixes #1056

Author: PR-Contributor

src/mcp/types/_types.py

@@ -1688,11 +1688,11 @@ class ElicitRequest(Request[ElicitRequestParams, Literal["elicitation/create"]])
 class ElicitResult(Result):
     """The client's response to an elicitation request."""
 
-    action: Literal["accept", "decline", "cancel"]
+    action: Literal["accept", "reject", "cancel"]
     """
     The user action in response to the elicitation.
     - "accept": User submitted the form/confirmed the action (or consented to URL navigation)
-    - "decline": User explicitly declined the action
+    - "reject": User explicitly rejected the action
     - "cancel": User dismissed without making an explicit choice
     """