fix(file source) Fix a data corruption bug with multi-char delimiters (#24028)

* Fix problem where multi-chars delimiters fail to be parsed if they happen right at a buffer boundary.

* Add changelog entry and remove debug println statements

* Update changelog.d/24027_file_source_multi_char_delimiter.fix.md

Co-authored-by: Thomas <thomasqueirozb@gmail.com>

* Add #[cfg(test)] to test method

* Expand comments

* Add a regression/integration test to file source

* Remove cfg test

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Co-authored-by: Thomas <thomasqueirozb@gmail.com>