Add Sign-In Service Client Configuration and Documentation

[anjolovic]

Description

This PR focuses on improving the developer setup experience and providing clear instructions for both client configuration and load testing preparation.

Changes

• Updated README.md with correct client configuration steps
• Added detailed SignIn::ClientConfig creation example
• Fixed typo in database seeding instruction
• Documented environment variable requirements
• Added load testing preparation guidance

Configuration Details

# Client Configuration
client = SignIn::ClientConfig.create!(
  client_id: 'sample_client_api',
  authentication: 'api',
  pkce: true,
  redirect_uri: 'http://localhost:4567/auth/callback',
  access_token_duration: 300,    # 5 minutes
  refresh_token_duration: 1800,  # 30 minutes
  access_token_audience: 'va.gov',
  service_levels: ['ial2'],
  access_token_attributes: ['first_name', 'last_name', 'email'],
  credential_service_providers: ['idme', 'logingov']
)

Environment Configuration

# .env.local
SIS_CLIENT_ID='sample_client_api'
SIS_BASE_URL='http://localhost:3000'
SIS_AUTH_TYPE='api'

Testing Instructions

1. Start vets-api server:

   cd vets-api
   rails s

2. Start the client:

   cd sign-in-service-client
   bin/setup
   bin/server

3. Visit http://localhost:4567 and verify:

   • Sign-in button works
   • Both ID.me and Login.gov options are available
   • Authentication flow completes successfully
   • Profile page displays user information

Load Testing Preparation

This configuration provides a foundation for load testing with:

• Defined token lifetimes
• IAL2 service level support
• Both ID.me and Login.gov providers
• API authentication mode with PKCE

Checklist

• Updated documentation
• Added client configuration
• Tested authentication flow
• Verified environment variables
• Prepared load testing foundation
• Added test coverage

Related Issues

Closes #[issue_number]

[bosawt]

Typo fix 👍

For the client configuration change, I think we should update/add the necessary seed in vets-api db/seeds/development.rb for load testing, and then potentially have this documentation in the load testing repository. I want to keep this application as simple as possible and not reveal the configuration more than necessary in this specific document (basically I want to keep this application as 'here's an example of a web OAuth client, here's an example of an api OAuth client', not 'here are all the possibilities you have access to with a Sign in Service client')