test: stabilize macOS downloader retry assertions

[zackees]

Fixes the macOS retry-test flake in #1187's main workflow. The final retry can surface as either a truncated-body or request-setup transport failure; both are retryable, and the behavioral contract is exhaustion of exactly five attempts.\n\nValidated locally:\n- soldr cargo fmt --all -- --check\n- soldr cargo test -p fbuild-packages-fetch truncated_bodies (4 passed)\n- soldr cargo clippy -p fbuild-packages-fetch --lib -- -D warnings\n- diff review: clean

Summary by CodeRabbit

• Bug Fixes
  • Improved retry behavior test coverage for interrupted downloads across different platforms and transport conditions.
  • Confirmed operations fail appropriately after the maximum retry attempts are exhausted.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between e07b5fe and 54231f6.

📒 Files selected for processing (1)

• crates/fbuild-packages-fetch/src/downloader.rs

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📝 Walkthrough

Walkthrough

Two retry-policy tests now avoid asserting platform-dependent truncated-body error text. They capture the final error and retain the stable assertion that retries exhaust after five attempts. No production download logic changed.

Changes

Retry test stability

Layer / File(s)	Summary
Retry exhaustion assertions crates/fbuild-packages-fetch/src/downloader.rs	Truncated-body and streaming retry tests no longer require a specific body-read error message and document that body-read or connection failures may occur on the final attempt.

Estimated code review effort: 1 (Trivial) | ~3 minutes

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/macos-downloader-retry-assertion

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