Enable SwiftLint rule: flatmap_over_map_reduce

[mokagio]

Summary

Enables SwiftLint's flatmap_over_map_reduce rule.

The rule prefers flatMap { ... } over map { ... }.reduce([], +) for flattening nested arrays — this avoids materialising an intermediate array of arrays.

• See commit message for the violation count and any rewrites.
• The change is type-preserving — local build deferred to CI.

Part of the Orchard SwiftLint rollout campaign.

Test plan

• CI build is green.
• swiftlint lint --strict --no-cache is clean against the rule.

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[dangermattic]

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	32105
	Version	PR #25529
	Bundle ID	org.wordpress.alpha
	Commit	21dcf30
	Installation URL	58p85nihdnof0

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	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	32105
	Version	PR #25529
	Bundle ID	com.jetpack.alpha
	Commit	21dcf30
	Installation URL	7bklpau3dosdg

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[Copilot]

Pull request overview

This PR enables SwiftLint’s flatmap_over_map_reduce rule and updates the one in-repo occurrence to match the preferred flattening pattern, avoiding the intermediate array-of-arrays allocation.

Changes:

• Enabled the flatmap_over_map_reduce rule in .swiftlint.yml.
• Replaced map { ... }.reduce([], +) with flatMap { ... } in GutenbergFilesAppMediaSource.getDocumentTypes(...).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
WordPress/Classes/ViewRelated/Gutenberg/Utils/GutenbergFilesAppMediaSource.swift	Updates array-flattening implementation to use flatMap per new SwiftLint rule.
.swiftlint.yml	Adds flatmap_over_map_reduce to the opt-in only_rules list.

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