docs: Clarify authentication options for cloud providers (Bedrock, Vertex AI, Foundry), fix broken documentation links

[acomarcho]

Problem

The current GitHub Action supports Microsoft Foundry but the documentation suggests OIDC is the only authentication method. However, Azure Foundry also supports API key authentication (ANTHROPIC_FOUNDRY_API_KEY), which is simpler to set up and doesn't require Azure login.

Additionally:

• The Microsoft Foundry documentation link is broken (points to old docs.anthropic.com URL)
• The cloud providers documentation incorrectly states that Bedrock, Vertex, and Foundry use "OIDC authentication exclusively" when all three support static credential options

Proposed Changes

1. Explicitly pass ANTHROPIC_FOUNDRY_API_KEY environment variable in action.yml (following the pattern used for AWS and Vertex AI credentials)
2. Fix broken documentation link to point to https://code.claude.com/docs/en/microsoft-foundry
3. Update cloud-providers.md to accurately document all supported authentication methods for each provider

Expected Outcome

Users should know they can authenticate with Microsoft Foundry (or via other clouds that Claude supports) using a simple API key without needing to set up OIDC.