feat: implement Auth0 identity provider with device flow support and add tests

[jgarverick]

This pull request adds comprehensive unit tests for both the Auth0 and Okta identity providers, significantly improving test coverage for authentication scenarios in the CLI. Additionally, it extends the Auth0Config model to support specifying an access token file, aligning its configuration options with Okta.

Improvements to authentication provider test coverage:

• Added a new test suite Auth0IdentityProviderTests covering various scenarios for Auth0 authentication, including token source selection, claim parsing, error handling, and support for groups, roles, and scopes. (src/aria-cli/Aria.Cli.Tests/Auth0IdentityProviderTests.cs)
• Added a new test suite OktaIdentityProviderTests covering Okta authentication scenarios, including token retrieval from environment/file, claim precedence, scope and group parsing, and error conditions. (src/aria-cli/Aria.Cli.Tests/OktaIdentityProviderTests.cs)

Configuration enhancements:

• Extended the Auth0Config model to support an optional AccessTokenFile property, allowing tokens to be loaded from a file in addition to environment variables. (src/aria-auth-core/Models/AriaConfig.cs)

[Copilot]

Pull request overview

Adds a real Auth0 identity provider implementation (including device flow) and significantly expands CLI authentication test coverage, while also aligning Auth0 configuration options with Okta via an access_token_file setting.

Changes:

• Implement Auth0 identity provider with env/file token resolution, JWT claim normalization, and device flow token acquisition.
• Improve Okta JWT parsing error handling (invalid base64 → clearer exception).
• Add new xUnit suites for Okta/Auth0 identity resolution behavior and update CLI docs to describe provider configuration/behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
src/aria-cli/Services/OktaIdentityProvider.cs	Wraps base64 decode failures with a clearer InvalidOperationException for invalid JWT payloads.
src/aria-cli/Services/Auth0IdentityProvider.cs	Implements Auth0 provider: token source resolution, device flow HTTP interactions, and JWT claim normalization.
src/aria-cli/README.md	Documents available auth providers, configuration examples, and claim normalization behavior.
src/aria-cli/Aria.Cli.Tests/OktaIdentityProviderTests.cs	Adds unit tests for Okta token source selection and claim parsing behavior.
src/aria-cli/Aria.Cli.Tests/Auth0IdentityProviderTests.cs	Adds unit tests for Auth0 token source selection and claim parsing behavior.
src/aria-auth-core/Models/AriaConfig.cs	Extends Auth0Config with optional access_token_file.