feat: Add public key fallback for SSH host key negotiation

[clement0010]

Changes

• Register the gateway's public key as a fallback host key in GetDownstreamConfig, alongside the existing certificate-based host key
• Allows SSH clients that don't support certificate-based host key algorithms (e.g. Claude Desktop) to connect using public key verification

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.39%. Comparing base (b61a9ec) to head (9d914dd).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
+ Coverage   82.26%   82.39%   +0.12%     
==========================================
  Files          35       35              
  Lines        2532     2533       +1     
==========================================
+ Hits         2083     2087       +4     
+ Misses        341      337       -4     
- Partials      108      109       +1     

Flag	Coverage Δ
integration	48.38% <100.00%> (+0.06%)	⬆️
unit	78.20% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/sshhandler/config.go	79.10% <100.00%> (+0.15%)	⬆️

... and 1 file with indirect coverage changes

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[minhtule]

LGTM 👍