go.mod and test hygiene

Four small hygiene fixes flagged in review:

• go.mod pinned go 1.26.2 in the go directive, forcing that exact patch toolchain on everyone. Use go 1.26 for the language requirement plus a toolchain go1.26.2 line.
• ExampleClient_Sync ran with no timeout (the review flagged real network I/O during go test). The repo's golangci-lint config enforces testableexamples, so the example must keep its // Output: directive and remain runnable; instead of dropping it, bound the call with a 30s context deadline so it can't hang. The example hosts are RFC 2606 reserved names that don't resolve, so it fails fast and prints nothing.
• The smart-HTTP test server's handlers called s.tb.Fatalf from the server's goroutines, where Fatalf's runtime.Goexit doesn't stop the test correctly. Use s.tb.Errorf (safe from any goroutine); each call is the handler's final statement, so the handler still returns.
• SyncRequest.Validate and PlanRequest.Validate were byte-for-byte duplicates; extract validateSyncFields and have both delegate.