test(auth): stop TestJWT_TamperedToken passing a valid signature

The test overwrote the last character of the signature with "X". An
HMAC-SHA256 signature is 32 bytes, so its base64url encoding is 43
characters and the final one carries only four significant bits — U, V, W
and X all decode to the same trailing byte. Roughly one token in sixteen
was therefore left byte-identical and validly signed, and the test failed
because ValidateToken correctly accepted it.

Measured at 3098/50000 (6.2%) over distinct signatures; it just failed the
Go job on this branch for reasons unrelated to the branch. Flipping a
character in the middle of the signature is 0/50000.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/auth/jwt_test.go

@@ -181,8 +181,20 @@ func TestJWT_TamperedToken(t *testing.T) {
 		t.Fatalf("GenerateAccessToken() error: %v", err)
 	}
 
-	// Tamper with the token by modifying the last character of the signature.
-	tampered := token[:len(token)-1] + "X"
+	// Tamper with the token by flipping a bit in the middle of the signature.
+	//
+	// Not the last character: an HMAC-SHA256 signature is 32 bytes, so its
+	// base64url encoding is 43 characters and the last one carries only four
+	// significant bits. U, V, W and X all decode to the same trailing byte, so
+	// overwriting the last character with "X" left roughly one token in
+	// sixteen byte-identical and validly signed — a real 6% flake, measured
+	// over 50k distinct signatures.
+	middle := len(token) - 20
+	flipped := byte('A')
+	if token[middle] == flipped {
+		flipped = 'B'
+	}
+	tampered := token[:middle] + string(flipped) + token[middle+1:]
 
 	_, err = svc.ValidateToken(tampered)
 	if err == nil {