Cas Fallback Telemetry

cas: fallbacks now emit telemetry

When the cas experiment is enabled and a CAS operation cannot complete, Terragrunt falls back to a slower path (the standard download client, or a temporary clone when the shared git store is unavailable) and keeps going. Until now the only record of a fallback was a warning in the logs, which made it impractical to measure how often CAS degrades across a fleet.

Each fallback now also emits a cas_fallback telemetry event whose reason attribute identifies the cause: init_error, getter_error, git_store_unavailable, probe_failure, or stack_generation_error. Operators collecting OpenTelemetry traces or metrics from Terragrunt can count and alert on these events to judge CAS health before relying on it by default.