Backport(v1.19): test_fluentd: fix flaky zero_downtime_restart from per-chunk log parsing (#5450)

[github-actions]

Which issue(s) this PR fixes:
Backport #5450
Fixes #

What this PR does / why we need it:
The read loop in run_fluentd split each eager_read chunk on its own (logs = buf.split("\n")) and yielded that to the caller for counting.
A single read can end in the middle of a line -- this happens when the source-only buffer is replayed and a whole chunk of records is flushed to stdout in one burst while the reader catches the pipe mid-write, which is much more likely on a loaded CI runner.
When a record's line straddles two reads, neither half matches the record regexp, so the caller drops it from its count while the full line is still present in the accumulated output.

Carry the incomplete trailing line across reads and yield only complete lines, the same way assert_log_matches already reads against the accumulated buffer.

This PR will fix following error:

4) Failure: test: should restart with zero downtime (no data loss)(TestFluentdCommand::zero_downtime_restart)
/Users/runner/work/fluentd/fluentd/test/command/test_fluentd.rb:1574:in `block (2 levels) in <class:TestFluentdCommand>'
     1571:         end
     1572:       end
     1573: 
  => 1574:       assert_equal(
     1575:         [(0..499).to_a, (0..499).to_a, (0..499).to_a],
     1576:         [
     1577:           records_by_type["udp"].sort,
<[[0,
  1,
  2,
  3,
  4,
  5,
...

Ref. https://github.com/fluent/fluentd/actions/runs/29796336422/job/88528337017#step:6:5375

Docs Changes:
N/A

Release Note:
N/A