fix: make LiteLLM API key optional for self-hosted proxies

[roomote-v0]

Related GitHub Issue

Closes: #11918

Description

This PR attempts to address Issue #11918. Feedback and guidance are welcome.

LiteLLM supports and is often used without authentication in self-hosted or local environments. Currently the extension UI enforces a mandatory API Key for the LiteLLM provider, preventing users from saving configurations or using the provider without entering a dummy key.

This change makes the API key field optional across the stack:

• webview-ui/src/utils/validate.ts: Changed validation to require only litellmBaseUrl (not litellmApiKey) for the LiteLLM provider
• webview-ui/src/components/settings/providers/LiteLLM.tsx: Updated handleRefreshModels to only require url and removed the litellmApiKey check from the Refresh Models button disabled condition
• src/shared/api.ts: Changed the LiteLLM type from { apiKey: string; baseUrl: string } to { apiKey?: string; baseUrl: string }
• src/core/webview/webviewMessageHandler.ts: Changed the condition from if (litellmApiKey && litellmBaseUrl) to if (litellmBaseUrl) so model fetching works without an API key
• src/api/providers/fetchers/litellm.ts: Changed the apiKey parameter type to string | undefined (the Authorization header is already conditionally included only when an API key is provided)
• src/api/providers/lite-llm.ts: Updated fallback from "dummy-key" to "not-needed" for clarity
• webview-ui/src/i18n/locales/en/settings.json: Added validation.baseUrl translation key

Test Procedure

• Ran the existing litellm test suite (22 tests pass) including 2 new tests for undefined/empty API key scenarios
• All lint checks pass
• All type checks pass across the monorepo

New tests added:

1. Verifies requests are sent without Authorization header when apiKey is undefined
2. Verifies requests are sent without Authorization header when apiKey is empty string

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue
• Scope: Changes are focused on the linked issue
• Self-Review: Performed a thorough self-review
• Testing: New tests added for the optional API key scenario
• Documentation Impact: No documentation updates required
• Contribution Guidelines: Read and agree to the Contributor Guidelines

Documentation Updates

• No documentation updates are required.

Additional Notes

The getLiteLLMModels fetcher already had conditional logic to only add the Authorization header when an API key is present (line 21-23). This change simply allows the rest of the stack to take advantage of that existing behavior.

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