feat: add Microsoft Foundry provider support (#684)

* feat: add Azure AI Foundry provider support

Add support for Azure AI Foundry as a fourth cloud provider option alongside Anthropic API, AWS Bedrock, and Google Vertex AI.

Changes:
- Add use_foundry input to enable Azure AI Foundry authentication
- Add Azure environment variables (ANTHROPIC_FOUNDRY_RESOURCE, ANTHROPIC_FOUNDRY_API_KEY, ANTHROPIC_FOUNDRY_BASE_URL)
- Support automatic base URL construction from resource name
- Add validation logic with mutual exclusivity checks for all providers
- Add comprehensive test coverage (7 Azure-specific tests, 3 mutual exclusivity tests)
- Add complete Azure AI Foundry documentation with OIDC and API key authentication
- Update README to reference Azure AI Foundry support

Features:
- Primary authentication via Microsoft Entra ID (OIDC) using azure/login action
- Optional API key authentication fallback
- Custom model deployment name support via ANTHROPIC_DEFAULT_*_MODEL variables
- Clear validation error messages for missing configuration

All tests pass (25 validation tests total).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* refactor: rename Azure AI Foundry to Microsoft Foundry and remove API key support

- Rename all references from "Azure AI Foundry" to "Microsoft Foundry"
- Remove ANTHROPIC_FOUNDRY_API_KEY support (OIDC only)
- Update documentation to reflect OIDC-only authentication
- Update tests to remove API key test case

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* docs: simplify Microsoft Foundry setup and remove URL auto-construction

- Link to official docs instead of duplicating setup instructions
- Remove automatic base URL construction from resource name
- Pass ANTHROPIC_FOUNDRY_BASE_URL as-is

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

docs/cloud-providers.md

@@ -1,16 +1,17 @@
 # Cloud Providers
 
-You can authenticate with Claude using any of these three methods:
+You can authenticate with Claude using any of these four methods:
 
 1. Direct Anthropic API (default)
 2. Amazon Bedrock with OIDC authentication
 3. Google Vertex AI with OIDC authentication
+4. Microsoft Foundry with OIDC authentication
 
 For detailed setup instructions for AWS Bedrock and Google Vertex AI, see the [official documentation](https://docs.anthropic.com/en/docs/claude-code/github-actions#using-with-aws-bedrock-%26-google-vertex-ai).
 
 **Note**:
 
-- Bedrock and Vertex use OIDC authentication exclusively
+- Bedrock, Vertex, and Microsoft Foundry use OIDC authentication exclusively
 - AWS Bedrock automatically uses cross-region inference profiles for certain models
 - For cross-region inference profile models, you need to request and be granted access to the Claude models in all regions that the inference profile uses
 
@@ -40,11 +41,19 @@ Use provider-specific model names based on your chosen provider:
     claude_args: |
       --model claude-4-0-sonnet@20250805
     # ... other inputs
+
+# For Microsoft Foundry with OIDC
+- uses: anthropics/claude-code-action@v1
+  with:
+    use_foundry: "true"
+    claude_args: |
+      --model claude-sonnet-4-5
+    # ... other inputs
 ```
 
-## OIDC Authentication for Bedrock and Vertex
+## OIDC Authentication for Cloud Providers
 
-Both AWS Bedrock and GCP Vertex AI require OIDC authentication.
+AWS Bedrock, GCP Vertex AI, and Microsoft Foundry all support OIDC authentication.
 
 ```yaml
 # For AWS Bedrock with OIDC
@@ -97,3 +106,36 @@ Both AWS Bedrock and GCP Vertex AI require OIDC authentication.
   permissions:
     id-token: write # Required for OIDC
 ```
+
+```yaml
+# For Microsoft Foundry with OIDC
+- name: Authenticate to Azure
+  uses: azure/login@v2
+  with:
+    client-id: ${{ secrets.AZURE_CLIENT_ID }}
+    tenant-id: ${{ secrets.AZURE_TENANT_ID }}
+    subscription-id: ${{ secrets.AZURE_SUBSCRIPTION_ID }}
+
+- name: Generate GitHub App token
+  id: app-token
+  uses: actions/create-github-app-token@v2
+  with:
+    app-id: ${{ secrets.APP_ID }}
+    private-key: ${{ secrets.APP_PRIVATE_KEY }}
+
+- uses: anthropics/claude-code-action@v1
+  with:
+    use_foundry: "true"
+    claude_args: |
+      --model claude-sonnet-4-5
+    # ... other inputs
+  env:
+    ANTHROPIC_FOUNDRY_BASE_URL: https://my-resource.services.ai.azure.com
+
+permissions:
+  id-token: write # Required for OIDC
+```
+
+## Microsoft Foundry Setup
+
+For detailed setup instructions for Microsoft Foundry, see the [official documentation](https://docs.anthropic.com/en/docs/claude-code/microsoft-foundry).