[CI] macOS checkout fetch ends with early EOF and loses credentials on retry

## Summary

Two macOS Sonoma checkout jobs failed before repository build or test tasks with the same Git transfer and retry-authentication sequence.

- Stable signature: `git-fetch-early-eof-credentials-lost`
- Classification: CI infrastructure failure
- Distinct monitored PRs: 2
- Confirmed job occurrences: 2
- First confirmed by the monitor: 2026-07-22T15:59:55Z
- Latest confirmed occurrence: 2026-08-01T03:43:24Z

## Occurrences

- #26235, head `2d6de8eaf12aff5c2f4bff984ce5085b1fece6f7`
- [Build 14731906, attempt 1, X64 Mac Sonoma job](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14731906&view=logs&jobId=623bc498-120b-5ff2-984a-5b073efffe1b)
- Checkout failed with an unexpected sideband disconnect, `fatal: early EOF`, and `invalid index-pack output`; task retries then could not read a GitHub username because terminal prompts were disabled.
- Retry outcome: Sonoma attempt 2 succeeded at 2026-07-22T16:29:53Z. The retained timeline also shows a later Sonoma attempt 3 succeeding.
- #26384, head `d45f335fdc821987a0d0086ebd976c6efb545806`
- [Build 14840799, attempt 1, X64 Mac Sonoma job](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14840799&view=logs&jobId=623bc498-120b-5ff2-984a-5b073efffe1b)
- Checkout fetched for about 62 minutes, then logged `unexpected disconnect while reading sideband packet`, `fatal: early EOF`, and `fatal: fetch-pack: invalid index-pack output` in task log 2883.
- All task-level retries then failed with `could not read Username for https://github.com: terminal prompts disabled`.
- Build 14840799 completed failed at 2026-08-01T03:47:30Z.

## Reproduction pattern and impact

The macOS checkout performs a large full-ref fetch. When that transfer disconnects after a long download, the retry path reinitializes the repository but no longer has usable GitHub credentials. The job never reaches repository build or test tasks.

## Investigation performed

The Azure build metadata, timelines, checkout task logs, and retry outcomes were inspected. Separate all-state issue searches used the exact sideband-disconnect, early-EOF, invalid-index-pack, credential, and checkout component strings. The only candidate, #10251, concerns product build performance and is unrelated.

No TestSummary or HtmlReport diagnostic is applicable because checkout failed before tests started.

## Retry status

- #26235 recovered on a later Sonoma retry.
- #26384 was not retried. Its completed run also contains deterministic expired-certificate failures, including Windows Integration, and replacement signing credentials are not validated. Retrying the failed set would repeat those failures, and Windows Integration requires an entire-stage retry.

## Current diagnosis and workaround

This is a CI checkout/network failure followed by a broken retry-authentication path. Investigate Git transfer reliability for the macOS agent and ensure checkout retries preserve or reacquire the GitHub authentication header. A fresh checkout attempt can recover, but #26384 must not be retried until the separate signing credentials are validated.

Azure logs and artifacts are retention-limited.

Start with the Azure build metadata and checkout task logs for builds 14731906 and 14840799, focusing on the macOS full-ref fetch and its retry path. Trace how the retry handles the GitHub authentication header after early EOF and invalid index-pack errors. Done means the checkout retry preserves or reacquires authentication and the failure pattern is addressed without retrying the unrelated signing-credential failures.

Written by the indexing model from the issue text.