feat(config): require merged top-level defaults

[dcoric]

Summary

Fixes #1287.

This updates config loading so user override files can keep using the generated optional GitProxyConfig type, while the merged runtime configuration is narrowed to a stricter FullGitProxyConfig after defaults are applied and verified.

Changes

• Add a FullGitProxyConfig type for the merged runtime configuration.
• Validate that default-backed top-level config values are present after merging defaults and user overrides.
• Keep deprecated compatibility fields optional where the default config does not set them.
• Remove fallback values from config accessors that now read from the validated merged config.
• Remove the duplicate session max-age fallback in service session setup.
• Add tests covering defaulted top-level values and partial user overrides.

Validation

• npm run check-types:server
• npm test -- test/testConfig.test.ts
• Commit hook ran prettier, eslint fixes, and npm run lint

[netlify]

✅ Deploy Preview for endearing-brigadeiros-63f9d0 canceled.

Name	Link
🔨 Latest commit	e5f1d82
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.78%. Comparing base (6981427) to head (e5f1d82).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1514      +/-   ##
==========================================
+ Coverage   90.21%   90.78%   +0.56%     
==========================================
  Files          69       69              
  Lines        5511     5543      +32     
  Branches      944      939       -5     
==========================================
+ Hits         4972     5032      +60     
+ Misses        521      494      -27     
+ Partials       18       17       -1     

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