Make explicit that GOOGLE_APPLICATION_CREDENTIALS is required in env vars for secops

[dandye]

In docs, we have:

      "command": "uvx",
      "args": [
        "secops-mcp"
      ],
      "env": {
        "CHRONICLE_PROJECT_ID": "your-project-id",
        "CHRONICLE_CUSTOMER_ID": "01234567-abcd-4321-1234-0123456789ab",
        "CHRONICLE_REGION": "us"
      },

we need:

      "command": "uvx",
      "args": [
        "secops-mcp"
      ],
      "env": {
        "CHRONICLE_PROJECT_ID": "your-project-id",
        "CHRONICLE_CUSTOMER_ID": "01234567-abcd-4321-1234-0123456789ab",
        "CHRONICLE_REGION": "us",
        "GOOGLE_APPLICATION_CREDENTIALS": "/Users/dandye/.config/gcloud/application_default_credentials.json"
      },

The call to SecOpsClient() in server.py hangs, so the issue presents to the user as a prompt that never returns and times out after value set in timeout. So, we should also investigate how to make noise (i.e. log an error) when this issue occurs.

I have more notes on the issue to add soon, but want to stub ASAP.

[mihirvala08]

In #179, we added support for Service Account credentials via SECOPS_SA_PATH. If this variable is provided, we no longer need GOOGLE_APPLICATION_CREDENTIALS.

We should update the logic to ensure that at least one of these environment variables (SECOPS_SA_PATH or GOOGLE_APPLICATION_CREDENTIALS) is present.