feat: expose ProviderRegistry.list()

[pachara-n]

What & why

This PR adds ProviderRegistry.list() and updates cmdList() to derive the provider list from the registry instead of using a hardcoded string.

This keeps the CLI output in sync with registered providers and allows runtime-registered providers to be reflected correctly.

Changes

• Added ProviderRegistry.list()
• Updated cmdList() to use the registry
• Added tests covering built-in and custom providers

Checklist

• Focused change with a clear description
• Ran format / lint / tests (see README)
• Updated docs or CHANGELOG if relevant
• Linked any related issue (Closes evalgate list hardcodes the provider names instead of asking the registry #3)

[github-actions]

Welcome, @pachara-n, and thanks for your first pull request to Evalgate.

A quick look at CONTRIBUTING.md covers how things are set up and run here.

I'll review this shortly. Ask anything in the thread if something is unclear.

[royalpinto007]

Merged, thanks @pachara-n. Deriving the list from the registry means the CLI can no longer drift from what is actually registered, which was the real bug behind the hardcoded string. Verified locally: 68 tests, typecheck and lint all clean.

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