refactor(libsy): use conversation IR request and response types

[nachiketb-nvidia]

Why

PR #17 introduced libsy with bespoke LlmRequest / LlmResponse structs that only carried a text prompt and completion. That makes libsy look like it owns a new LLM wire abstraction, when Switchyard already has a shared provider-neutral conversation IR in switchyard-translation.

This change makes libsy align with that direction now: LlmRequest and LlmResponse are semantic aliases over ConversationRequest and ConversationResponse, while provider-specific details stay in clients/hosts rather than in router algorithms.

What

• Replaces libsy-owned LlmRequest and LlmResponse structs with aliases to ConversationRequest and ConversationResponse.
• Adds small text helpers for text-only router logic and examples: text_request, text_response, request_text, and response_text.
• Updates llm_class, ensemble, and rand to inspect conversation IR via the helpers instead of old prompt / completion fields.
• Boxes Step::ReturnToAgent so the larger conversation response does not inflate the stream enum.
• Updates libsy examples and README to teach the conversation IR contract.
• Updates the libsy proxy demo to the current Algorithm / Request / Response API and compiles it against the shared router builder.

How

The algorithms still operate on plain text where their current logic requires text, but that text is now extracted from the shared IR. Generated subcalls are built as text-only ConversationRequests with the inbound model preserved in model; routed target selection remains on Decision::selected_model().

The proxy demo still lets libsy make routing decisions and Switchyard own HTTP/profile serving. The demo now creates a text-only libsy request from inbound prompt text, runs the classifier algorithm directly, and returns a chat-completion response for Switchyard to translate back to the inbound format.

What to Review

• Whether aliasing LlmRequest / LlmResponse to the translation IR is the right libsy contract for this PR.
• Whether the helper functions are the right minimum bridge for text-based router algorithms without reintroducing a second IR.
• Whether boxing Step::ReturnToAgent is acceptable for the stream API after the response type becomes larger.
• Whether the demo proxy update is appropriately scoped to compiling against the current libsy/server APIs.

Validation

• cargo test -p libsy
• cargo check -p libsy --examples
• cargo check -p libsy-proxy
• cargo test -p libsy-proxy
• pre-commit hook: cargo fmt, cargo clippy

[messiaen]

#17 is merged can you rebase? @nachiketb-nvidia

[messiaen]

Let's move these types into a protocol or similar crate.