wolfsshd: expand AuthorizedKeysFile %u/%h/%% tokens per user

[yosuke-wolfssl]

wolfsshd: expand AuthorizedKeysFile %u/%h/%% tokens per user

Problem

ResolveAuthKeysPath copied an absolute AuthorizedKeysFile pattern into the
resolved path verbatim, with no token substitution (a TODO marked the gap).
A common OpenSSH-migration config such as:

AuthorizedKeysFile /etc/ssh/keys/%u

resolved to the literal /etc/ssh/keys/%u for every user. If a file existed
at that un-substituted path, its keys were accepted for any valid system user,
collapsing per-user key isolation.

Addressed by f_5827.

Fix

• Add ExpandAuthKeysTokens() — expands %u (user), %h (home dir), and %%
  (literal %) into a bounds-checked buffer. Unrecognized tokens and
  over-length expansions fail closed (WS_FATAL_ERROR) rather than reaching
  the filesystem.
• ResolveAuthKeysPath() now takes the connecting user and expands the pattern
  before the absolute/relative decision, so both branches honor substitution and
  each user resolves to a distinct path.
• Thread the username through SearchForPubKey() → CheckPublicKeyUnix (Unix)
  and CheckPublicKeyWIN (Windows). Both call sites already held it; no new
  getpwnam/lookup is introduced and no shared mutable state is added.

Token semantics

Token	Expands to
%u	connecting user
%h	user home dir
%%	literal %

A recognized token with no value (e.g. %u with a NULL user) and any
unrecognized token fail closed. A trailing lone % is treated as a literal.

Platform notes

Absolute-path detection is /-rooted on POSIX; on Windows it additionally
recognizes a \-root or a drive-letter (X:) root, guarded by #ifdef _WIN32
so a POSIX relative pattern beginning with \ or <letter>: is not misread.

Testing

• New test_ResolveAuthKeysPath unit test (13 scenarios): per-user %u
  (alice vs bob), %h, %%, relative, default-NULL fallback, trailing %,
  unrecognized token, NULL-user fail-closed, expansion over-length, relative
  home-dir over-length, and platform-aware drive-letter/backslash handling.
• MAX_PATH_SZ moved to configuration.h so the test buffer tracks the real
  contract; ResolveAuthKeysPath exposed to the tests via WOLFSSHD_STATIC.
• Verified under ASan + UBSan (clean), with negative controls confirming the
  tests catch both the verbatim-copy regression and a removed bounds check.
• Native Windows MSVC build (VS Build Tools 2022, x64): auth.c /
  configuration.c compile with no new warnings; default test set
  (api-test, unit-test, testsuite) passes.

Files changed

• apps/wolfsshd/auth.c
• apps/wolfsshd/auth.h
• apps/wolfsshd/configuration.h
• apps/wolfsshd/test/test_configuration.c

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[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1064

Scan targets checked: wolfssh-bugs, wolfssh-src

No new issues found in the changed files. ✅