feat(runner): support embedded Rust hosts

[afourniernv]

What

Adds AlgorithmSpec::build_with_runtime_models(...), which builds an algorithm and its matching runtime model groups from the same configuration.

Adds LlmClientError::Host so an embedded Rust host can preserve its native error without treating it as an upstream or FFI failure. Host error details are kept out of Switchyard telemetry.

Why

Embedded gateways could already build an AlgorithmSpec, but the runtime model-group mapping was private. A host such as LiteLLM would otherwise need to duplicate Switchyard's algorithm-specific model mapping.

The existing string and FFI error variants also could not preserve a typed native host error with the right semantics. These two additive surfaces let LiteLLM run Switchyard routing while continuing to own provider execution.

Notes for reviewers

Start with crates/switchyard-runner/src/algorithm.rs, then crates/protocol/src/client.rs.

No existing public methods or signatures are changed.

Validated with:

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace