fix: replace panic with Result in LANCE_INITIAL_UPLOAD_SIZE validation

[LuciferYang]

Summary

• Replace panic!() with Result error in initial_upload_size() when LANCE_INITIAL_UPLOAD_SIZE env var is set to an out-of-range value (<5MB or >5GB), preventing process crashes from misconfiguration
• Store the validated upload_size in the ObjectWriter struct to eliminate repeated OnceLock lookups and avoid any unwrap/expect in the upload path
• Extract MAX_UPLOAD_PART_SIZE constant for the 5GB upper bound

Motivation

Setting LANCE_INITIAL_UPLOAD_SIZE to a value outside the valid range (less than 5MB or greater than 5GB) crashes the entire process via panic!(). This is a disproportionate response to a configuration error — the caller has no opportunity to handle the failure gracefully. Since ObjectWriter::new() already returns Result<Self>, propagating the error is natural and idiomatic.

What Changed

initial_upload_size(): Return type changed from usize to Result<usize>. Range violations now return Err(Error::invalid_input(...)) instead of panicking. Non-numeric values still silently fall back to the 5MB default, consistent with sibling env vars (LANCE_UPLOAD_CONCURRENCY, LANCE_CONN_RESET_RETRIES).

ObjectWriter struct: Added upload_size: usize field, set once in new() from the validated result. This eliminates repeated OnceLock access in next_part_buffer() and removes the need for any unwrap/expect in the hot path.

next_part_buffer(): Takes upload_size: usize as a parameter instead of calling initial_upload_size() internally.

Behavioral Equivalence

Input	Before	After
Env not set	Returns 5MB default	Ok(5MB)
Non-numeric (e.g. "abc")	Silent fallback to 5MB	Ok(5MB) (unchanged)
Valid integer in range	Returns the value	Ok(value)
Integer < 5MB	panic!()	Err(...)
Integer > 5GB	panic!()	Err(...)

No other behavior changes. All existing callers of ObjectWriter::new() already operate in Result context.

Test plan

• cargo test -p lance-io -- object_writer — all 4 tests pass
• cargo clippy -p lance-io -- -D warnings — no warnings
• cargo fmt -p lance-io -- --check — no formatting issues
• cargo check --workspace --tests — full workspace compiles

[codecov]

Codecov Report

❌ Patch coverage is 67.64706% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-io/src/object_writer.rs	67.64%	9 Missing and 2 partials ⚠️

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