build: upload GutenbergKit source maps to Sentry

[dcalhoun]

Description

GutenbergKit editor exceptions currently surface in Sentry with minified,
unreadable JS frames (e.g. editor-CZ39QhNu.js:1:326003, see
JETPACK-IOS-1KAQ). This makes
them hard to triage.

GutenbergKit produces the editor JS but the host app publishes the exceptions to
Sentry, so the host must upload GutenbergKit's source maps — exactly as we
already do for the React Native (gutenberg-mobile) bundle via
upload_gutenberg_sourcemaps. GutenbergKit now publishes its source maps
alongside its XCFramework
(GutenbergKit#554);
this PR fetches and uploads them.

How it works

• New upload_gutenberg_kit_sourcemaps fastlane helper resolves the
  GutenbergKit version from Modules/Package.resolved, downloads the matching
  GutenbergKitSourceMaps.zip from the CDN, and uploads it to Sentry.
• Because the editor JS runs inside the host app, its exceptions carry the host
  app's release/dist, so the maps upload under the same
  <app_identifier>@<release_version>+<build_version> coordinates as the RN
  maps, with rewrite/strip_common_prefix.
• Wired into the same App Store, prototype, and TestFlight lanes that already
  upload the RN source maps.
• The version resolver also handles a pr-build/<N> snapshot-branch pin
  (mapping to gutenbergkit/pr-builds/<N>/) so the pipeline can be verified
  against a GutenbergKit PR build before a release exists.
• The upload is best-effort: a GutenbergKit version without published maps is
  logged and skipped rather than failing the build.

This is the iOS half of a cross-repo effort; a follow-up will add the equivalent
to WordPress-Android.

Note

Depends on GutenbergKit#554
(produces + publishes the maps). Draft until that lands and end-to-end
symbolication is verified against a prototype build.

Testing instructions

Verified locally: rubocop passes and fastlane lanes loads with no errors.
The version resolver was unit-tested against both a released-version pin and a
pr-build/<N> branch pin.

End-to-end verification (in progress): pin GutenbergKit to PR #554's
pr-build/554 branch, run a prototype build, trigger a GutenbergKit editor
exception, and confirm the resulting Sentry event de-minifies. sentry-cli sourcemaps explain <event-id> confirms the release/dist match.

[dangermattic]

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	33267
	Version	PR #25800
	Bundle ID	org.wordpress.alpha
	Commit	6da41d9
	Installation URL	2uv4tgjbek4rg

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📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	33267
	Version	PR #25800
	Bundle ID	com.jetpack.alpha
	Commit	6da41d9
	Installation URL	2og17ikh8oq5g

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