Fail fast on missing Azure credentials for abs:// litestream replica URL

[Copilot]

The ABS://*) case in check_litestream_replica_url was empty, allowing misconfigured deployments to pass entrypoint validation and only fail later when litestream attempts to connect to Azure Blob Storage.

Changes

• Credential fast-fail for abs://: Validates that at least one Azure auth mechanism is present before the container starts:

  1. Account key — LITESTREAM_AZURE_ACCOUNT_KEY
  2. Service principal — all three of AZURE_CLIENT_ID + AZURE_TENANT_ID + AZURE_CLIENT_SECRET
  3. Managed identity — IDENTITY_ENDPOINT (injected by Azure Container Apps / Functions runtimes)

• Consistent failure propagation: Uses the same ||-chain idiom as require_env_var, so log_error → false propagates under set -e and the container exits immediately rather than continuing.

ABS://*)
    env_var_is_defined "LITESTREAM_AZURE_ACCOUNT_KEY" \
        || { env_var_is_defined "AZURE_CLIENT_ID" && env_var_is_defined "AZURE_TENANT_ID" && env_var_is_defined "AZURE_CLIENT_SECRET"; } \
        || env_var_is_defined "IDENTITY_ENDPOINT" \
        || log_error "Azure Blob Storage ('abs://') requires at least one auth mechanism: ..."
    ;;