refactor(build): consolidate integrationTest sources into test

[rstata]

This issue document commit a51f7d8 from Airbnb, which didn't have a good commit message:

Merges the src/integrationTest source set into src/test for the
tenant/api, tenant/codegen, and tenant/runtime modules, removing
the separate integration-test source set from both the Gradle and Bazel
builds.

Motivation

The integrationTest source set was a historical artifact — the tests
it contained were ordinary JUnit tests that ran in the same JVM as the
unit tests and had no need for a separate source set. Consolidating into
src/test simplifies the build configuration and eliminates the
conceptual overhead of maintaining two parallel test trees.

Changes

Gradle

• Removed the integrationTest source set and the
  integration-test.gradle.kts convention plugin that wired it up.
• Merged integrationTest Gradle task dependencies into the standard
  test task.
• Updated build.gradle.kts files for tenant/api, tenant/codegen,
  and tenant/runtime accordingly.

Bazel

• Moved all src/integrationTest source files into src/test.
• Updated all affected BUILD.bazel files so each package's test
  library carries the full union of deps that were previously split
  across the unit-test and integration-test BUILD files.
• Packages that used the feature_app_codegen macro
  (globalid, execution) now invoke it directly from their
  src/test BUILD file, preserving the same generated-GRT behaviour.
• Fixed a hardcoded src/integrationTest path reference in
  viaduct_classdiff_test_macro.bzl.
• Fixed SchemaClassFileDiff to load the default Viaduct schema from
  the correct classpath path in both Gradle (default_schema.graphqls
  at the classpath root) and Bazel (via a new genrule +
  java_library target that renames and re-roots the generated
  @@viaduct_default_schema.graphqls file).