docs: Clarify cloud provider auth options and fix Foundry documentation

[acomarcho]

Closes #768

Summary

• Add explicit ANTHROPIC_FOUNDRY_API_KEY environment variable pass-through in action.yml files
• Fix broken Microsoft Foundry documentation URL
• Update cloud-providers.md to accurately document authentication options

Changes

1. Add ANTHROPIC_FOUNDRY_API_KEY support

Added ANTHROPIC_FOUNDRY_API_KEY: ${{ env.ANTHROPIC_FOUNDRY_API_KEY }} to both action.yml and base-action/action.yml, following the existing pattern for AWS and Vertex AI credentials.

2. Fix documentation link

Updated broken link from https://docs.anthropic.com/en/docs/claude-code/microsoft-foundry to https://code.claude.com/docs/en/microsoft-foundry.

3. Update authentication documentation

Updated docs/cloud-providers.md to accurately reflect that:

• AWS Bedrock supports IAM access keys, OIDC (temporary credentials), or bearer token
• Google Vertex AI supports service account key files or OIDC via Workload Identity Federation
• Microsoft Foundry supports API key or OIDC via Azure login

Added a new section showing the simpler API key authentication setup for Microsoft Foundry.

Test plan

• Tested Microsoft Foundry API key authentication in production workflows using Claude Opus 4.5 for pull request reviews at my company