[MCP Elicitation] Content validation should be done only for accept action

[vnalawad-tibco]

Describe the bug
When elicitation request is canceled or declined, current implementation still attempts to validate content against schema

To Reproduce

1. Register MCP server (with tool sending elicitation request) in GitHub Copilot
2. Enter prompt that would trigger MCP server tool
3. When elicitation request received, cancel it

Expected behavior
In such scenario, SDK call(ServerSession.Elicit(...)) expected to return response back with cancel action.
Instead, implementation attempts to validate elicitation response.

Logs

Additional context
I believe validating content against schema only for accept action type should solve this issue.