fix: skip priming events for clients with old protocol versions

Priming events (SEP-1699) have empty SSE data which older clients cannot
handle - they try to JSON.parse("") and crash. Only send priming events
to clients with protocol version >= 2025-11-25, which includes the fix
for handling empty SSE data.

For the initialize request, the protocol version is extracted from the
request params. For subsequent requests, it's taken from the
mcp-protocol-version header.

Author: felixweinberger