cow-venue: type the retry classifier boundary with OrderbookApiErrorType

[mfw78]

What

The retry classifier boundary becomes typed. classify and is_already_submitted now take cowprotocol::OrderbookApiErrorType instead of &str, and the adapter passes api.error_kind() instead of &api.error_type. The Unknown(_) variant is handled explicitly at the call site (an unlisted wire errorType is by definition permanent, so it drops).

Why

OrderbookApiErrorType already exists (with an Unknown(String) fallback) and the crate's parity test already trusts it, but the hot path stayed stringly, so a call site could pass an arbitrary or misspelt &str with no compile-time signal. The generated table may stay string-backed internally (matched via as_str()), but nothing untyped now crosses the classifier boundary.

Testing

Full battery on the pinned 1.94 toolchain, all green:

• nix develop --command just ci (fmt, clippy -D warnings, doc, module wasms, cargo test --workspace --all-features)
• nix develop --command just build-cow-venue
• nix develop --command just check-venue-agnostic
• nix develop --command just check-cow-orderbook-only

AI Assistance

Implemented with Claude Code.

Closes #496