fix(bindings/python): fix sized File.readline

[Xuanwo]

Which issue does this PR close?

Part of #8159.

Rationale for this change

File.readline(size) initializes its destination buffer with size zero bytes. BufRead::read_until appends the line after those bytes, and the implementation then truncates the buffer using only the appended length. As a result, sized reads return zero bytes while advancing the file cursor.

What changes are included in this PR?

Read sized lines into an empty buffer while retaining the existing take(size) limit. Add a synchronous regression test against io.BytesIO that covers partial lines, newline boundaries, zero-sized reads, cursor progression, and EOF.

Are there any user-facing changes?

Yes. File.readline(size) now returns the bytes read from the file, up to the requested size, instead of zero-filled data.

AI Usage Statement

AI materially assisted with root-cause analysis, implementation, regression-test construction, and local validation. Validation used the memory backend through the shared blocking reader path; the full remote-service behavior matrix was not run.