Closes #52489

The special token check in `prompt_input_contains_special_tokens` used
`String::contains()` to look for `=======\n` in the buffer. This meant
any line containing `=======` (like `// =======` section separators)
would cause edit predictions to bail out entirely.

Fixed by only matching when the token appears at the start of a line,
since the git merge markers are always placed at line boundaries in the
prompt.

Added tests for both the helper function and a regression test for the
reported issue.

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Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>