Add common utils CI gate

[jormundur00]

Fixes #910

What changed

This PR adds a dedicated CI gate for common/utils.

Before this change, the repository had dedicated CI coverage for other common-module ownership boundaries, but common/utils did not have its own workflow gate.

After this change, pull requests touching common/utils can trigger a focused workflow that runs :utils:checkstyleMain, :utils:checkstyleTest, and :utils:test.

Why

common/utils is shared infrastructure. Giving it its own ownership-boundary CI gate makes failures easier to localize and keeps repository CI structure consistent with the other common modules.

Example

Before:

A PR changing common/utils relied on broader CI coverage and did not have a dedicated workflow responsible for the module's own checks.

After:

A PR touching common/utils can trigger test-common-utils.yml, which runs the focused utils checks for that module.

Implementation summary

• Add .github/workflows/test-common-utils.yml to run :utils:checkstyleMain, :utils:checkstyleTest, and :utils:test on relevant pull request paths and manual dispatch.
• Update CI architecture and common executable-test docs to declare the new common-utils gate.
• Remove the external network dependency from FileUtilsTest by replacing live HTTP URLs with deterministic in-memory HttpURLConnection fixtures.
• Rename SchemaValidationUtilsTest methods to satisfy Checkstyle while keeping display names.

Validation

• grund check
• grund fmt . --marker --check
• JAVA_HOME=/home/jovan/.sdkman/candidates/java/17.0.12-graal ./gradlew :utils:checkstyleMain :utils:checkstyleTest :utils:test --rerun-tasks
• git diff --cached --check