[processing] Fix save vector features to file algorithm's handling of pre-existing files

[nirvn]

Description

This PR harmonize handling of preexisting files by the save vector features to file algorithm by informing users of default action (file overwrite) and allow to switch to other action/behavior (keep file, overwrite layer, keep file and layer, etc.).

Fixes #54333 .

[DelazJ]

@nirvn the [not selected] entry in the screenshot... is a leftover from previous design? It is not an option, right?

[qgis-bot]

@nirvn
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
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[qgis-bot]

@nirvn

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[nyalldawson]

Suggested change

	outputs.insert( QStringLiteral( "FILE_PATH" ), filePath );
	outputs.insert( QStringLiteral( "FILE_PATH" ), finalFileName );

[nirvn]

@nyalldawson , addressed.

[nirvn]

@DelazJ , it was an "option" until just now. The "[Not selected]" item is automatically added as the first option of an optional enum parameter. That's a hard-coded QGIS processing behavior.

That said, I've just made the enum non-optional after a discussion with Nyall. While we needed the enum parameter to be optional to avoid API breakage, other forthcoming changes in the way QGIS processing handles things will improve things on that front.

[github-actions]

@nirvn
A documentation ticket has been opened at qgis/QGIS-Documentation#8486
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!

[dlejay]

@nirvn Why have you chosen “overwrite file” as default behaviour ?

[nirvn]

@dlejay , to retain previously established behavior as the default one.

[agiudiceandrea]

Hi @nirvn, please see #56517.