feat(protocol): preserve embedded host errors

[afourniernv]

What

Adds LlmClientError::Host for errors owned by an embedded Rust host.

The original error stays boxed so the host can recover its native type. Switchyard records it as a host error without putting the source text into telemetry.

Why

An embedded client sometimes needs to pass a host error through Switchyard and recover it at the outer boundary. General(String) loses the type, while Ffi describes a different boundary.

Notes for reviewers

Start with crates/protocol/src/client.rs. The remaining changes keep the new category safe and consistent in libsy, client-call telemetry, and runner failure summaries.

This adds one variant to the existing #[non_exhaustive] LlmClientError enum. No existing methods, variants, or signatures change. The error is terminal unless the host maps it to one of Switchyard's existing retryable categories.

Related: #912 adds a separate runner-construction API for embedded hosts. The two PRs can merge independently.

Validated with:

• cargo fmt --all --check
• cargo test -p switchyard-libsy boxed_sources_are_reduced_to_their_class
• cargo test -p switchyard-llm-client --test observability host_error_source_is_redacted_from_the_client_call_span
• cargo test -p switchyard-runner failure::tests
• cargo test -p switchyard-llm-client fallback_only_accepts_context_policy_and_unavailable_failures