build(e2eTest): give :e2eTest friend-path access to :auth internals

[demolaf]

Wires :e2eTest's Kotlin compilations as friends of :auth, so the e2e tests can drive test seams that are internal. Unblocks narrowing the credential-manager seams before the GA cut.

Changes

• Add a friendPaths block to e2eTest/build.gradle.kts that filters the compile classpath (libraries) for entries under auth/build, rather than hardcoding the AGP intermediates jar it resolves to today.
• Use rootProject.layout.projectDirectory.dir("auth/build") rather than project(":auth").layout.buildDirectory, which works but is cross-project model access that breaks under project isolation.

Two approaches that do not work

Appending -Xfriend-paths to compilerOptions.freeCompilerArgs is ignored, because KGP generates that flag itself from the typed friendPaths property. Pointing at auth/build/tmp/kotlin-classes/debug also fails, because :e2eTest never sees that directory. Both were measured.

AGP 9.0 caveat

This wiring hangs off org.jetbrains.kotlin.gradle.tasks.KotlinCompile, so the built-in-Kotlin migration the build already warns about will need it rewritten.

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Maintainer note: Fixes internal CPRN-492

[gemini-code-assist]

Code Review

This pull request configures the e2eTest module to have friend access to :auth internals by adding the :auth build directory to the friendPaths of Kotlin compilation tasks. The review feedback suggests avoiding the use of rootProject to maintain Gradle's Project Isolation principles, and recommends using File.startsWith(File) instead of string-based path matching to prevent false positives with similarly named directories.

[gemini-code-assist]

Using rootProject in a subproject's build script violates Gradle's Project Isolation principles, as it couples the subproject directly to the root project model at configuration time. Instead, you can navigate relatively using the current project's layout.projectDirectory.\n\nAdditionally, using startsWith on absolute path strings can lead to false positives if there are other directories with similar prefixes (e.g., auth/build-cache). Using Kotlin's File.startsWith(File) extension is safer as it compares actual path components.

val authBuildDir = layout.projectDirectory.dir("../auth/build").asFile\ntasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {\n    friendPaths.from(libraries.filter { it.startsWith(authBuildDir) })\n}

[demolaf]

Collapsed into #2529, which now carries both the friend-path wiring and the narrowing against pre-GA. On its own this change was unfalsifiable: nothing in :auth is internal at this commit, so the only thing that demonstrates the wiring works is the narrowing compiling against it.