fix: Prevent block_locator from ignoring newlines

[slg95]

Description

block_locator was trimming empty lines from the beginning and end of files. This is more impactful when trying to append to a file that ends in a newline, which would include any text file adhering to the POSIX standard (ending in a newline).

Sometimes LLMs include the trailing newline in the search block, which causes the bound check to short-circuit in SearchEngine:_evaluate_position().

If you want to try to reproduce the Issue, I would suggest creating a simple file such as:

Hello World

then telling an LLM to append something to the end of the file. I had this action fail with Claude Sonnet.

• I've read the contributing guidelines and have adhered to them in this PR
• I've updated the README and/or relevant docs pages
• I've run make test to ensure all tests pass
• I've run make format to format the code
• I've run make docs to update the vimdoc pages

[ravitemer]

Thanks for raising this. This should be fixed now.