fix: flush accumulated buffer on EOF without trailing newline

[Vitalymt]

Problem

When a file doesn't end with a newline character, the last line is silently dropped.

The read loop in worker.go accumulates data in accumBuf until it finds a \n. At EOF, the remaining data is saved to job.tail without ever being emitted through controller.In(). If the file is never written to again, that data is lost permanently.

Steps to reproduce:

1. Create a file with content that doesn't end with \n: echo -n "hello" > /tmp/test.txt
2. Configure file.d to read this file
3. The "hello" event is never emitted

Fix

Emit accumBuf as a final event before calling processEOF when EOF is reached and the buffer is non-empty. Clear both accumBuf and job.tail afterward to prevent re-emission on file truncation.

Changes:

• plugin/input/file/worker.go: +8 lines — flush buffer on EOF
• plugin/input/file/worker_test.go: updated existing test expectation + added multi-line test case

Fixes #912