GeneratePinCode off-by-one loop condition returns short PIN ~16% of the time, causing intermittent pairing failure

Summary

GeneratePinCode(pinLength) in utils/src/dm_random.cpp:101 returns pinLength - 1 characters approximately 1/pinLength of the time (~16% for 6-digit PINs). This causes device pairing to fail intermittently with no clear error.

Vulnerable Code

// utils/src/dm_random.cpp:101
// left_digit_count = pinLength - 1
while (pinCode.length() < left_digit_count) {   // '<' allows exact-length exit

When std::to_string(gen()) appends a sum that brings the total to exactly pinLength - 1, the loop exits one iteration early. substr(0, pinLength) on a shorter string silently returns fewer characters.

Example (pinLength=6)

Initial:  GenRandInt(1,9) = "3"           → length 1
Iter 1:   gen() = "7254"                  → length 5
Check:    5 < 5? FALSE → exit             ← too early
Result:   substr(0, 6) on 5-char string → "37254"  ← 5 chars, not 6

Downstream validation rejects this: "37254".length() < DM_MIN_PINCODE_SIZE (6) → pairing fails.

Trigger Conditions

1. GeneratePinCode is called with pinLength (typically 6)
2. Random generation produces a sum that brings pinCode.length() to exactly left_digit_count (pinLength - 1)
3. Loop condition pinCode.length() < left_digit_count evaluates false, exits early
4. substr(0, pinLength) on the shorter string returns a PIN with fewer characters
5. Downstream check rejects the short PIN → device pairing fails

Impact

• Intermittent pairing failure: ~16% of 6-digit PIN generations produce a short PIN, causing device pairing to fail
• No clear error: The short PIN is silently generated; the failure manifests downstream, making diagnosis difficult
• Affected callers: ExportAuthInfo and any code path calling GeneratePinCode

Suggested Fix

- while (pinCode.length() < left_digit_count) {
+ while (pinCode.length() <= left_digit_count) {

substr(0, pinLength) already bounds the result, so overshooting is harmless. The <= condition ensures the loop always runs enough iterations.