test: make network auth fetch regression test deterministic

[bschellenberger2600]

Summary

Replace the external-network auth regression test with a local HTTP auth challenge so it validates non-interactive git behavior deterministically.

Why this change?

make test-race on current main fails in internal/git because the test depends on a GitHub URL whose failure mode changed to repository not found, so the test no longer reliably exercises the terminal-prompt-disabled path.

Bug Reproducibility (if this fixes a bug)

• Repro steps:
  1. Check out main at latest head after 3b65f5e.
  2. Run make test-race.
  3. Observe TestNetworkFetch_UnauthenticatedHTTPSFailsWithoutPrompt fail with unexpected auth/error text.
• If reproducibility steps are not feasible, explain why:

Tests

• I added or updated tests for all code changes in this PR.
• For bug fixes, I added regression coverage.
• I ran go test -race -count=1 ./....
• I ran go vet ./....

Risk / Rollback Notes (optional)

Low risk: only test code changed. Rollback by reverting this commit if needed.

  

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