fix(publishing): restore Maven Central publishing for isolated kits

[thomson-t]

Background

The 6.0.1 release workflow attempted to publish four Kotlin-isolated kits from
their standalone Gradle roots. Those builds could not access the root-only
buildSrc publication convention, so the Maven Central publication task was
not registered and the isolated artifacts were not published.

What Has Changed

• Move the Maven publication convention from buildSrc into shared
  build-logic that is available to root, aggregate-kit, and standalone-kit
  builds.
• Configure stable artifact IDs and exact release-version resolution for
  urbanairship-20, braze-43, ga-23, and ga4-23.
• Validate generated POMs for Maven Central publication and run Maven-local
  smoke tests before uploading release artifacts.
• Update the draft and publish workflows to validate every isolated kit before
  Maven Central uploads.
• Document the isolated-kit coordinates, development workflow, and
  post-release verification process.

Screenshots/Video

N/A — build and release workflow changes only.

Checklist

• Self-review completed
• Tests added or updated
• Tested locally

[cursor]

PR Summary

Medium Risk
Changes affect release-time Maven publishing and CI validation for four isolated kits; runtime SDK behavior is unchanged, but a misconfiguration could block or mis-publish release artifacts.

Overview
Restores Maven Central publishing for Kotlin-isolated kits (urbanairship-20, braze-43, ga-23, ga4-23) by wiring those standalone Gradle roots into shared build-logic via includeBuild, applying mparticle.android.library.publish, and pinning stable mparticleMavenPublish.artifactId / VERSION resolution.

Release safety: POM validation now runs before Maven Central uploads (not only mavenLocal), and Release - Draft / Release - Publish add Maven-local smoke tests for aggregate kits plus each isolated kit. Isolated Central publish steps switch from -PisRelease=true to -Pmparticle.kit.mparticleFromMavenLocalOnly=true.

Docs and README tables are updated for the new coordinates and publishMavenPublicationToMavenLocal workflow; build-logic drops redundant AGP/Kotlin classpath deps.

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[cursor]

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