build: add 2026.1 build target

[aseemxs]

Problem

Add IntelliJ 2026.1 (build prefix 261) build target support using EAP SDK 261.22158-EAP-CANDIDATE-SNAPSHOT.

Changes

• Added 2026.1 IDE profile to IdeVersions.kt (Community, Ultimate, Gateway, Rider)
• Updated build script mappings (BuildScriptUtils.kt, kotlinResolution.settings.gradle.kts)
• Added 261 to OAuth collaboration modules bundling (toolkit-intellij-subplugin.gradle.kts)
• Added 2026.1 to intellij.rd.platform bundled module (jetbrains-ultimate/build.gradle.kts)
• Removed com.jetbrains.codeWithMe from 261 ultimate profile (plugin removed in 261)
• Split CwmProblemsViewMutator.kt into version-specific dirs (src-251-253/ original, src-261+/ no-op stub — BackendToolWindowHost removed in 261)
• Added 2026.1 to CI version matrix and run configurations

Tests

• Compilation passes clean
• 77 test failures due to Byte Buddy not yet supporting Java 25 (JBR 25 ships with 261 EAP). Investigating options: upgrading Byte Buddy/Mockito or adding -Dnet.bytebuddy.experimental=true to test JVM args for 261.

[github-actions]

Qodana for JVM

1 new problem were found

Inspection name	Severity	Problems
Extension class should be final and non-public	🔶 Warning	1

💡 Qodana analysis was run in the pull request mode: only the changed files were checked

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