Tests | Add repo-server source matrix

[dag-andersen]

Summary

• Add a repo-server source matrix organized around the decisions buildManifestRequestForSource makes: STR/TYP/REACH/REF/REQ/TRV/NEG
• Add coverage for strategy selection, source-type detection, file reachability, ref handling, request invariants, app-of-apps traversal, and negative cases
• Add known-bug repro tests for remaining uncovered rows, skipped by default unless RUN_KNOWN_BUG_TESTS=1

Why this helps

• Turns the repo-server render path into an explicit compatibility contract instead of relying on user-reported edge cases
• Pins regressions for recently fixed repo-server-api issues
• Documents repo-server behavior discovered from the Argo CD implementation
• Leaves executable repros for the known gaps so each can be enabled when fixed

Tests

• make run-unit-tests
• golangci-lint run ./pkg/reposerverextract/...
• RUN_KNOWN_BUG_TESTS=1 go test -run 'TestSourceMatrix_(TYP5|REACH5|REACH6|REACH11|REF13)' ./pkg/reposerverextract/... -v - confirms 5 opt-in repros fail