fix: clear stale tail buffer after log rotation

[immanuwell]

If a tailed file ends with a partial line and gets rotated or recreated, the old prefix leaks into the first line of the new file. You get stale-prefixfresh line. Pretty easy to hit in the wild with container log rotation.

This clears the buffered prefix on rotate, truncate, and recreate. Normal append still keeps partial lines, so that bit stays the same.

Repro

1. Start TailReader
2. Write stale-prefix without \n
3. Rotate or recreate the file
4. Write fresh line\n
5. Before: stale-prefixfresh line
6. After: fresh line

Tested with go test tail_reader.go tail_reader_test.go -run TestTailReader -count=3

[def]

@immanuwell could you please clarify whether you've actually encountered this issue in your environment?