openssh: add sshconfig_fuzz support and authkeys_fuzz seed corpus

[XananasX7]

Summary

Two improvements to the openssh OSS-Fuzz project:

1. Wire in sshconfig_fuzz (SSH client config parser)

A new harness sshconfig_fuzz.cc has been added upstream (openssh/openssh-portable#680) covering readconf.c — the SSH client configuration file parser. This file is 3957 lines and handles complex, security-sensitive directives:

• Match blocks with multi-criteria evaluation
• ProxyCommand / ProxyJump argument parsing
• IdentityFile / CertificateFile path expansion
• Hostname canonicalization rules
• Per-host option inheritance and merging

This parser is exercised every time ssh reads ~/.ssh/config but has had no OSS-Fuzz coverage until now.

The build.sh already auto-discovers all *_fuzz.cc harnesses via:

FUZZER_TARGETS=$(cd regress/misc/fuzz-harness && ls *_fuzz.cc | grep -v sntrup761 | ...)

This PR adds the seed corpus generation for sshconfig_fuzz.

2. Add missing authkeys_fuzz seed corpus

authkeys_fuzz.cc exists in the openssh repo and is listed in the upstream Makefile's TARGETS, but the build.sh had no seed corpus setup for it. This PR adds inline seed generation:

• Plain authorized_keys entry
• Entry with from=, command= restrictions
• Entry with no-pty,no-x11-forwarding,no-agent-forwarding options
• Testdata keys when present in the source tree

Testing

Seed corpus generation uses only mkdir, printf, cat, and zip — same pattern as other seed sections in this build.sh.

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[github-actions]

XananasX7 is a new contributor to projects/openssh. The PR must be approved by known contributors before it can be merged. The past contributors are: arthurscchan, hunsche, daztucker, djmdjm, DonggeLiu, inferno-chromium, cvediver, Dor1s

[XananasX7]

I have read the CLA Document and I hereby sign the CLA

[DavidKorczynski]

#15598 (review)

[XananasX7]

CLA is signed via the Google CLA portal. This PR adds a sshconfig_fuzz harness and authkeys seed corpus not currently in the integration. Happy to coordinate with existing maintainers if they'd prefer to absorb the changes directly.

[XananasX7]

Closing this PR — projects/openssh already exists in the oss-fuzz repository at https://github.com/google/oss-fuzz/tree/master/projects/openssh. The sshconfig_fuzz harness and authkeys seed corpus additions should be proposed against the existing integration. Apologies for the duplicate.