go.mod: raise go directive 1.19 -> 1.26.5

1 module(s) in this repo declared go < 1.22. Go 1.22 changed for-loop
variable scoping to per-iteration, so raising the directive past 1.22 can
change runtime behaviour. Each module here was scanned for loops whose
loop variable is captured by a func literal / goroutine / defer, or has its
address taken: zero such sites were found, so the bump is behaviour-preserving.

Verified per module with go build, go vet and go test (-mod=readonly).

Co-authored-by: Hanzo Dev <[email protected]>

go.mod

@@ -1,3 +1,3 @@
 module github.com/hanzoai/go-openrouter
 
-go 1.19
+go 1.26.5