fix(table): surface a malformed manifest size from DynamoDB

[jackylee-ch]

A manifest row's size attribute was read with .ok(), so a value that is not a number silently became None. The sibling version field, parsed from the same item seven lines below, reports the same failure with a descriptive error:

let size = item.get("size").and_then(|attr| match attr {
    AttributeValue::N(size) => size.parse().ok(),   // silently None
    _ => None,
});
...
let version = version.parse().map_err(|e| Error::invalid_input(format!(
    "dynamodb error: could not parse the version number returned {}, error: {}", version, e)))?;

Losing the size is not free either — read_manifest falls back to a HEAD request for the length, so a corrupt row costs an extra round trip on every read and gives no signal why.

Read the attribute in one place: an absent value stays None for rows written before the column existed, and a present one that is not a number is reported.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The shared parser preserves the legitimate legacy absence case while consistently rejecting malformed stored metadata in both DynamoDB lookup paths. This enforces the existing (path, size) contract without changing writers or requiring migration.