Channels can now pass HTTP headers to MCP servers on a per-request basis
by storing keys with a "mcp:" prefix in InboundContext.Raw. This enables
forwarding bearer tokens from channel authentication to MCP tool calls.
The context flows: Channel → InboundContext.Raw["mcp:Authorization"]
→ toolshared.MCPHeaders(ctx) → headerTransport.RoundTrip → HTTP header.
SSE/HTTP transports now always use headerTransport (previously only when
static headers were configured) so dynamic headers work out of the box.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
