Backport PR #16760 to 8.17: ensure inputSize state value is reset during buftok.flush

[github-actions]

Backport PR #16760 to 8.17 branch, original message:

---

The BufferedTokenizer.flush must reset the inputSize value back to zero on flush, otherwise it will grow until a buffer full exception is thrown if the buffer has a size limit.

We can see the test fails if the inputSize = 0 is not present:

❯ bin/rspec logstash-core/spec/logstash/util/buftok_spec.rb -fd -e flush
[...]
FileWatch::BufferedTokenizer
  flush
    emits the contents of the buffer
    resets the state of the buffer
    with decode_size_limit_bytes
      resets the state of the buffer (FAILED - 1)
      emits the contents of the buffer

Failures:

  1) FileWatch::BufferedTokenizer flush with decode_size_limit_bytes resets the state of the buffer
     Failure/Error: expect(subject).to be_empty
       expected `#<FileWatch::BufferedTokenizer:0x35977ba7>.empty?` to be truthy, got false
     # ./logstash-core/spec/logstash/util/buftok_spec.rb:65:in `block in <main>'
     # ./spec/spec_helper.rb:84:in `block in <main>'

But with it tests pass:

❯ bin/rspec logstash-core/spec/logstash/util/buftok_spec.rb -fd -e flush

FileWatch::BufferedTokenizer
  flush
    resets the state of the buffer
    emits the contents of the buffer
    with decode_size_limit_bytes
      emits the contents of the buffer
      resets the state of the buffer

Finished in 0.0246 seconds (files took 1.34 seconds to load)
4 examples, 0 failures

[elastic-sonarqube]

Quality Gate passed

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0 Fixed issues
0 Accepted issues

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[donoghuc]

Clean backport, green CI.