chore(deps): update go toolchain directive to v1.26.4

Author: renovate[bot]

go.mod

@@ -2,7 +2,7 @@ module github.com/datum-cloud/datum-mcp
 
 go 1.25.0
 
-toolchain go1.25.7
+toolchain go1.26.4
 
 require (
 	github.com/coreos/go-oidc/v3 v3.18.0