[8.19](backport #51570) filestream: Fix date sorter short path panic

[mergify]

Proposed commit message

Fix filestream date sorter short path panic

Return a zero timestamp when a rotated file path is shorter than the configured
date format so copytruncate sorting cannot panic inside the prospector
goroutine.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

None

How to test this PR locally

go test -race ./filebeat/input/filestream/...

Real reproduction:

rm -rf /tmp/fbr /tmp/fbr-home-fixed
mkdir -p /tmp/fbr /tmp/fbr-home-fixed
python - <<'PY'
from pathlib import Path
base = Path('/tmp/fbr')
for name, prefix in [('a', 'original'), ('a-1', 'rotated-one'), ('a-2', 'rotated-two')]:
    with (base / name).open('w') as f:
        for i in range(140):
            f.write(f'{prefix} line {i:03d} abcdefghijklmnopqrstuvwxyz\n')
PY
cat >/tmp/fbr-filebeat.yml <<'YAML'
filebeat.inputs:
  - type: filestream
    id: repro-copytruncate-datesort
    paths:
      - /tmp/fbr/*
    prospector.scanner.check_interval: 1s
    rotation:
      external:
        strategy:
          copytruncate:
            suffix_regex: '-\d$'
            dateformat: '2006-01-02T15:04:05.000000Z07:00'

output.console:
  enabled: true
  codec.format:
    string: 'EVENT: %{[message]}'

logging.level: debug
logging.to_stderr: true

queue.mem:
  flush.timeout: 0s
YAML

timeout 5 /tmp/filebeat-fixed -e --strict.perms=false -c /tmp/fbr-filebeat.yml --path.home=/tmp/fbr-home-fixed > /tmp/fbr-fixed.log 2>&1
grep 'File /tmp/fbr/a-2 is rotated' /tmp/fbr-fixed.log
grep 'EVENT:' /tmp/fbr-fixed.log
grep -E 'panic|slice bounds out of range' /tmp/fbr-fixed.log

On an unpatched build, the same config exits with a panic after detecting /tmp/fbr/a-2 as rotated:

panic: runtime error: slice bounds out of range [-20:]
github.com/elastic/beats/v7/filebeat/input/filestream.(*dateSorter).GetTs
github.com/elastic/beats/v7/filebeat/input/filestream.(*dateSorter).sort
github.com/elastic/beats/v7/filebeat/input/filestream.rotatedFilestreams.addRotatedFile
github.com/elastic/beats/v7/filebeat/input/filestream.(*copyTruncateFileProspector).onRotatedFile
github.com/elastic/beats/v7/filebeat/input/filestream.(*copyTruncateFileProspector).onFSEvent
github.com/elastic/beats/v7/filebeat/input/filestream.(*copyTruncateFileProspector).Run.func2
github.com/elastic/go-concert/unison.(*MultiErrGroup).Go.func1

---

This is an automatic backport of pull request #51570 done by [Mergify](https://mergify.com).

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @orestisfl? 🙏