Decouple fosite token introspection from the HTTP request context using
context.WithoutCancel + 10s timeout. When claude.ai opens multiple
concurrent MCP connections and one disconnects, the token validation
for subsequent connections no longer fails with "context canceled".
Fixes Bug #6 from #bugs-synapbus.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>