Append default suffix when saving/exporting with a single filter

[denisfalqueto]

Summary

On some Linux desktop environments, the native/portal file dialog used for
saving and exporting doesn't auto-append the file extension when the user
doesn't type one. This leaves the caller (e.g. PDF export in the main
MuseScore repo) unable to pick the correct writer for the returned path,
causing exports to silently fail.

• In makeSelectFileQuery() (interactive.cpp), when opening a SaveFile
  dialog, request a defaultSuffix from the dialog derived from the
  suggested current path's own suffix.
• This is only done when the caller offers a single filter. Some
  callers (e.g. "Save As" for a MuseScore project) offer two filters at
  once — the normal *.mscz format and an experimental uncompressed-folder
  option — and a dialog-wide default suffix would be applied regardless of
  which filter the user actually selected, silently defeating the
  alternate option. That ambiguous case is left untouched.

Companion PR

This is a companion fix to a related PR in musescore/MuseScore
(src/app/main.cpp and src/project/internal/exportprojectscenario.cpp),
which adds a safety net that enforces the correct suffix after the dialog
returns, regardless of dialog backend.

Testing

Verified with a full local Debug build of MuseScore Studio (main repo,
pointing at this branch) on KDE Plasma Wayland:

• Exporting to PDF without typing an extension now correctly produces a
  .pdf file.
• Exporting to PDF while typing .pdf explicitly still works, without a
  duplicated suffix.
• Keyboard interaction inside the file dialog (arrow keys, typing, Enter to
  confirm) verified unaffected — this was a deliberate regression check,
  since a past issue (#13113) was caused by a different, broken Linux file
  dialog swallowing keyboard input.

Disclaimer

This change was implemented with the assistance of Claude Code (Anthropic),
which performed the code changes, the build, and the testing steps
described above. A human reviewed and validated the entire process step by
step, and endorses the final result.

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 6ac4402 and db2cecd.

📒 Files selected for processing (1)

• framework/interactive/internal/interactive.cpp

📝 Walkthrough

Walkthrough

This change modifies the Linux implementation of the internal makeSelectFileQuery function used in the save-file flow. When exactly one filename filter is provided and the current path has a non-empty file suffix, that suffix is extracted and set as the query's defaultSuffix. The change is scoped to the SaveFile branch under the Linux platform condition, with no other modes or branches affected, and no public or exported declarations altered.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is detailed, but it omits the required Resolves issue line and the template checklist items.	Add a Resolves #NNNN line and complete the required checklist fields, including CLA, commits, testing, and unit test status.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: adding a default suffix for save/export dialogs with a single filter.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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