Function classify_embedding_error 

pub fn classify_embedding_error(err: AppError) -> FallbackReason

Classify an embedding AppError into a typed FallbackReason.

v1.0.85 (ADR-0043): discriminates the 4 new causes (SlotExhausted, OAuthQuota, BackendMismatch, DimZero) from the legacy generic EmbeddingFailed bucket. The classification is purely lexical (substring match on the message) — no I/O, no retries, no telemetry, deterministic and #[serial_test::serial(env)]-safe.