Trim trailing whitespace when writing settings.json

[ThanhNguyxn]

Summary of the Pull Request

This patch strips trailing spaces and tabs from each serialized line before writing settings.json. It keeps current JSON formatting options intact while preventing whitespace-only churn from Settings UI saves.

References and Relevant Issues

Closes #10887

Detailed Description of the Pull Request / Additional comments

• Added a small line-by-line sanitizer in CascadiaSettings::WriteSettingsToDisk.
• Applied the sanitizer to the serialized string returned by Json::writeString(...) before _writeSettingsToDisk(...) is called.
• Preserves line endings and only removes trailing horizontal whitespace at end-of-line.

Validation Steps Performed

• Verified issue triage constraints:
  • Issue Settings editor saves settings.json lines with trailing whitespace #10887 is open, unassigned, and has no linked PR.
  • No duplicate linkage on the issue timeline.
• Ran git diff --check to confirm no trailing whitespace/conflict markers in this patch.
• Attempted LSP diagnostics, but local environment cannot start LSP on Windows+Bun 1.3.5 (known toolchain limitation).

PR Checklist

• Closes Settings editor saves settings.json lines with trailing whitespace #10887
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

[ThanhNguyxn]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 19834 in repo microsoft/terminal

[DHowett]

I appreciate your eagerness to contribute, but I would much rather have somebody contribute a fix to our upstream JSON serialization library (tracked in open-source-parsers/jsoncpp#1154) or manage to import that pull request as a patch rather than introduce a hundred lines of our own string processing code to resolve what is--effectively--a visual glitch somebody just doesn't like. :)

[DHowett]

I've done that over here: #19836