Fix logs -n N truncating the oldest line at the read-chunk boundary

[chengboonrong]

Summary

container logs -n N truncated the oldest of the N returned lines whenever those
lines spanned more than one 1024-byte read chunk. For example, a single line
longer than 1024 bytes:

$ container logs -n 1 logbug | wc -c
1024        # expected ~3009; output also starts mid-line (leading "START" gone)

Root cause

Application.ContainerLogs.tail(fh:n:follow:) reads the file backward in
1024-byte chunks and stopped as soon as lines.count >= n. While offset > 0
the buffer starts in the middle of a line, so the first line in the buffer is a
fragment truncated at the chunk boundary — and Array(lines.suffix(n)) kept it.

Fix

Keep reading until there are more than n non-empty lines (so the first of
the n kept lines is guaranteed complete) or the start of the file is reached.
The backward read is extracted into a testable lastLines(fh:n:); the follow
and full-file paths are unchanged.

Tests

Adds ContainerLogsTests covering a single line longer than one chunk, N lines
spanning multiple chunks (the general case), the ordinary short-line case, and
N larger than the file. Verified locally: swift build,
swift format lint --strict, and the new tests all pass.

Fixes #1967

🤖 Generated with Claude Code