SSHD/Echoserver: Fix memory leaks and public-key lookup

[stenslae]

Fixed: Memory leaks, multiple public-key lookups per user in the echoserver, and authentication privilege errors.

• internal.c in internal.c: Fixed a memory leak where RSA/ECC keys in signature blocks were leaked if they matched X.509 format IDs instead of standard SSH format IDs.
• auth.c: Freed promptData.prompts and promptData.promptLengths on allocation failure pathways.
• echoserver.c: Fixed a copy-paste error and added cleanups on allocations failure.
• Key Hash Lookup Correctness: Updated wsUserAuth in echoserver.c to traverse all
  registered public keys for a matching user name instead of immediately returning an invalid
  key error on the first hash mismatch.
• Stack Buffer Cleanup: Freed keyLoadBuf under WOLFSSH_SMALL_STACK configurations for all early exit paths inside echoserver_test.
• auth.c: Returns success immediately if privilege separation is explicitly disabled.
• auth.c: Limits changing target user/group identifiers to environments where privilege separation or sandboxing is active.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1071

Scan targets checked: wolfssh-bugs, wolfssh-src

Findings: 1
1 finding(s) posted as inline comments (see file-level comments below)

This review was generated automatically by Fenrir. Findings are non-blocking.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1071

Scan targets checked: wolfssh-bugs, wolfssh-src

Findings: 1
1 finding(s) posted as inline comments (see file-level comments below)

This review was generated automatically by Fenrir. Findings are non-blocking.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1071

Scan targets checked: wolfssh-bugs, wolfssh-src

No new issues found in the changed files. ✅

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1071

Scan targets checked: wolfssh-bugs, wolfssh-src

No new issues found in the changed files. ✅