fix(downloader): preserve object sizes across replica failures

[shikhar]

Replica fallback can return HTTP 416 with a smaller or missing object size after an earlier replica reported a larger one. Keep the largest known size independently of error selection, so sequences such as 416(size=17) → 503 → 416(size=0 or unknown) retain 17 when the final error is a 416. Preserve existing error precedence and fallback behavior.

Document that this reports the largest observed size without establishing replica agreement. Regression tests cover size orderings, empty and unknown sizes, intervening backend errors, and error precedence. A scripted SDK connector also exercises the full downloader path with 416 response headers.

Closes #153

Validation:

• Confirmed all three new size-preservation regression tests fail before the fix.
• cargo nextest run --locked --all-features: 113 passed, 1 intentionally skipped; includes Docker integration tests.
• cargo +nightly fmt --check
• cargo clippy --locked --all-features --all-targets -- -D warnings --allow deprecated
• cargo deny check: passed with existing duplicate-dependency and unused-license-allowance warnings.

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Changes how object download failures track size information.

The PR appears safe to merge; no actionable issue was identified.

Summary

The PR preserves the largest size reported by observed unsatisfied-range responses across replica fallback, documents that the size does not imply replica agreement, and adds unit and downloader-path regression tests.

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