fix: preserve final line of config files lacking a trailing newline

[synfinatic]

GenerateNewFile dropped the last line of a file when it had no trailing newline, because bufio.Reader.ReadString returns a partial line with io.EOF together, and both copy loops exited before writing it.

Fixes #1419

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.31%. Comparing base (0a68b41) to head (99979a0).

Files with missing lines	Patch %	Lines
internal/fileutils/fileedit.go	50.00%	3 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1420      +/-   ##
==========================================
- Coverage   88.48%   88.31%   -0.16%     
==========================================
  Files          56       56              
  Lines        3767     3781      +14     
==========================================
+ Hits         3333     3339       +6     
- Misses        299      303       +4     
- Partials      135      139       +4     

Flag	Coverage Δ
unittests	88.31% <50.00%> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/fileutils/fileedit.go	70.59% <50.00%> (-3.69%)	⬇️

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[Copilot]

Pull request overview

Fixes a data-loss bug in FileEdit.GenerateNewFile where the final line of a config file could be dropped when the file lacks a trailing newline, ensuring unmanaged content outside the managed # BEGIN_AWS_SSO_CLI / # END_AWS_SSO_CLI block is preserved.

Changes:

• Update GenerateNewFile to write the final partial line returned alongside io.EOF (both before locating the managed block and when copying the unmanaged tail).
• Add unit tests covering config files that do not end with a trailing newline (both with and without an existing managed block).
• Document the fix in CHANGELOG.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
internal/fileutils/fileedit.go	Writes EOF partial lines so the final unmanaged line isn’t dropped when there’s no trailing newline.
internal/fileutils/fileedit_test.go	Adds regression tests for no-trailing-newline inputs.
CHANGELOG.md	Notes the bugfix under Unreleased.

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