Incorrect `ANTHROPIC_FOUNDRY_BASE_URL` instructions

# TL;DR

The docs for Foundry authentication should have the `ANTHROPIC_FOUNDRY_BASE_URL` variable set to `https://my-resource.services.ai.azure.com/anthropic`, not `https://my-resource.services.ai.azure.com`, or at least note that this is sometimes necessary.

# Bug report

**Describe the bug**
Docs on `ANTHROPIC_FOUNDRY_BASE_URL` format are incorrect.

**To Reproduce**
Copy the [PR review action](https://github.com/anthropics/claude-code-action/blob/main/docs/solutions.md#automatic-pr-code-review), then use the [Foundry authentication instructions](https://github.com/anthropics/claude-code-action/blob/main/docs/cloud-providers.md#oidc-authentication-for-cloud-providers), specifically the instructions to set the env variable `ANTHROPIC_FOUNDRY_BASE_URL: https://my-resource.services.ai.azure.com`.

After following these instructions, I get a "resource not found" error.

**Expected behavior**
Successful automatic code review.

**API Provider**

[ ] Anthropic First-Party API (default)
[ ] AWS Bedrock
[ ] GCP Vertex
[x] Azure Foundry