chore: upgrade go to 1.26.2

## What

Upgrade Go from 1.25.1 to 1.26.2 in go.mod.

## Why

go1.26.2 fixes 4 vulnerabilities in the standard library:

- GO-2026-4947: unexpected work during chain building (crypto/x509)
- GO-2026-4946: inefficient policy validation (crypto/x509)
- GO-2026-4870: unauthenticated TLS 1.3 KeyUpdate DoS (crypto/tls)
- GO-2026-4866: case-sensitive excludedSubtrees auth bypass (crypto/x509)

## Notes

- This is a two-minor-version jump (1.25 → 1.26); all tests pass

Signed-off-by: jmeridth <jmeridth@gmail.com>

Author: jmeridth

go.mod

@@ -1,6 +1,6 @@
 module github.com/privateerproj/privateer-sdk
 
-go 1.25.1
+go 1.26.2
 
 require (
 	github.com/gemaraproj/go-gemara v0.0.2