chore(deps): bump Go to 1.26.4 (GO-2026-5039, GO-2026-5037)

govulncheck flagged two stdlib vulns present in go1.26.3, both fixed in
go1.26.4: GO-2026-5039 (net/textproto unescaped input in errors) and
GO-2026-5037 (crypto/x509 inefficient hostname parsing). Bump the go
directive so setup-go provisions the fixed toolchain.

Author: kanywst

go.mod

@@ -1,6 +1,6 @@
 module github.com/0-draft/mcp-authzen
 
-go 1.26.3
+go 1.26.4
 
 require github.com/mark3labs/mcp-go v0.54.1