## Summary
- Switched variant dispatch from `ClientSession` RPC calls to direct
inner server handler invocation via a new `backendSession` type,
preserving `context.Context` values (e.g. `RequestContext`) that were
previously lost crossing the in-memory transport boundary
- Added `backendSession` abstraction in `session.go` that mirrors the
`mcp.ClientSession` API but calls the inner server's handler chain
directly, forwarding `RequestExtra` (token info, HTTP headers, etc.)
- Captured the inner server's handler chain via receiving middleware
(`captureMCPMethodHandler`) in `backend.go`, reused across all
connections
- Simplified `innerConnection` to hold `*backendSession` + cleanup;
in-memory transport/client kept alive only for notification forwarding
(progress, logging)
- Refactored `getSession` → `getConnection` returning `*innerConnection`
to reduce indirection in dispatch code
- Moved `injectVariantMeta` calls to after type assertion and nil checks
in all dispatch handlers for consistency and nil-safety
## Test plan
- [x] Existing variant dispatch tests pass (`go test ./variants/...`)
- [x] Verify context values (e.g. `RequestContext`) are available in
tool handlers when invoked through variant dispatch
- [x] Verify `RequestExtra` fields (token info, HTTP headers) are
forwarded to inner server handlers
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