fix(packages): retry response-body download failures

[zackees]

Summary

• retry complete buffered and streaming transfers up to five total attempts
• use exponential 1s, 2s, 4s, and 8s backoff for transient request, 5xx, body, and chunk-stall failures
• discard failed-attempt buffers and only write the destination after a complete transfer
• add paused-time real TCP regression coverage for success on attempt five and exhaustion without a sixth request

Validation

• soldr cargo test -p fbuild-packages-fetch --lib downloader (14 passed)
• soldr cargo test -p fbuild-packages-fetch --lib (122 passed)
• soldr cargo clippy -p fbuild-packages-fetch --all-targets -- -D warnings
• soldr cargo check --workspace --all-targets
• workspace unit suite: downloader tests pass; unrelated fbuild-python --test pyo3_policy failure reproduces unchanged on main

Closes #1038

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 32e8abea-a5f1-452b-8273-c181c6251580

📥 Commits

Reviewing files that changed from the base of the PR and between 2705844 and 89fbe93.

📒 Files selected for processing (2)

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

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/1038-download-retries

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