Return UploadPlan struct from upload_parameters

[demoray]

calc_concurrency and upload_parameters in src/upload/blobstore.rs
returned a positional (NonZeroU64, NonZeroUsize). Both values are
non-zero positive integers; nothing at the call site or in the tests
prevented a swapped destructure beyond the type difference, and that
protection disappears the first time anyone aligns the two on a
single integer type.

Introduce a private UploadPlan { block_size, concurrency } so the
single call site (BlobUploader::upload_file) reads field-named:

let plan = upload_parameters(file_size, self.block_size, self.concurrency)?;
BlobClientUploadOptions {
    parallel: Some(plan.concurrency),
    partition_size: Some(plan.block_size),
    ..Default::default()
}

Rewrite the five plan-returning tests to assert against a whole
UploadPlan literal instead of two positional bindings plus two

No behavior change. Public API unchanged — UploadPlan and
upload_parameters are both private.

Verified with:

• cargo fmt --check
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test --all-features