feat(misk-testing): add WebActionRegistrationTesting utility

[rnc505]

Description

A common mistake when developing Misk services is implementing a WebAction but forgetting to register it via WebActionModule.create<YourAction>(). Without registration, the action won't be exposed as an HTTP endpoint, and this can be difficult to catch during development.

This PR adds WebActionRegistrationTesting to misk-testing - a utility that scans a service's packages for WebAction implementations and verifies each has a corresponding WebActionEntry registration. When missing registrations are detected, it fails with a helpful error message including copy-paste registration code.

What's included:

• WebActionRegistrationTesting - core utility with assertAllWebActionsRegistered()
• AbstractWebActionRegistrationTest - JUnit base class for easy adoption
• Documentation in docs/actions.md under a new "Verifying action registration" section
• ClassGraph dependency for classpath scanning

Example usage:

class WebActionRegistrationTest : AbstractWebActionRegistrationTest() {
  override fun createInjector() = Guice.createInjector(MyServiceTestingModule())
  override fun webActionPackages() = listOf("com.example.myservice")
}

Testing Strategy

• Added unit tests in WebActionRegistrationTestingTest.kt covering:
  • Passing when all actions are registered
  • Failing with helpful message when actions are missing
  • Excluding abstract classes automatically
  • Respecting custom exclude predicates
  • Including registration module hint in error messages
• Verified build passes with gradle :misk-testing:build

Communication Plan

• This is an additive, opt-in feature with no breaking changes
• Services can adopt it incrementally by adding a single test class

Checklist

• [ ] I have reviewed this PR with relevant experts and/or impacted teams.
• [x] I have added tests to have confidence my changes work as expected.
• [x] I have a rollout plan that minimizes risks and includes monitoring for potential issues.