feat(switchyard-translation): add neutral-IR encode/decode helpers

[messiaen]

Summary

Adds convenience helpers to switchyard-translation for translating between provider wire
formats and the neutral IR without threading an engine/policy through every call.

Base: libsy-lib-proto-cleanup

Changes

• helpers.rs — free functions over a shared default engine: decode_request,
  encode_request, decode_buffered_response, encode_buffered_response, encode_stream,
  and a byte-based decode_stream (Stream<Item = Result<Vec<u8>, _>> → IR chunks — no HTTP
  client types), plus the RawEventStream type for unframed wire-event streams.
• sse.rs — a minimal SSE frame parser backing decode_stream.
• lib.rs — wires in the two modules and re-exports the helpers from the crate root.
• Cargo.toml — adds futures + async-stream for the streaming helpers.

Tests

Unit tests for every helper (request/response round-trips, buffered & stream encode/decode)
and the SSE parser. Full workspace builds clean, clippy at zero.

Checklist

• One class per file; filename = snake_case of the primary class.
• New public symbols exported from switchyard/__init__.py.__all__ if intended for downstream use.
• Unit tests added for new components / bug fixes.
• README / --help updated if customer-facing surface changed.
• Commits signed off (Signed-off-by: Your Name <email>) per the DCO.

Notes for reviewers

Anything reviewers should pay extra attention to — risky paths, follow-up tickets, intentional trade-offs.

Summary by CodeRabbit

• New Features
  • Added convenient request and response translation between supported wire formats.
  • Added streaming translation for real-time responses, including SSE event encoding and decoding.
  • Added support for buffering partial stream frames and recognizing stream completion markers.
  • Preserved model information when encoding buffered responses.
• Bug Fixes
  • Improved handling of incomplete UTF-8 data and non-standard streams that omit a final separator.

[coderabbitai]

Walkthrough

Changes

The translation crate adds buffered request/response helpers, streaming encode/decode APIs, SSE frame parsing, associated tests, public re-exports, and futures-based runtime dependencies.

Translation streaming

Layer / File(s)	Summary
SSE frame parsing crates/switchyard-translation/src/sse.rs	Adds buffered SSE boundary detection, UTF-8 decoding, JSON and terminal-frame parsing, and parser tests.
Buffered and streaming translation helpers crates/switchyard-translation/src/helpers.rs	Adds request/response wrappers and stream conversion helpers that preserve translation state, handle terminal events, and support incomplete trailing frames with tests.
Crate integration and runtime support crates/switchyard-translation/src/lib.rs, crates/switchyard-translation/Cargo.toml	Registers the new modules, re-exports helper APIs, and adds futures and async-stream dependencies.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Poem

A rabbit hops through streams of light,
Translating chunks from day to night.
SSE frames neatly fall in line,
JSON carrots taste just fine.
“Done!” I twitch—then rest my ear.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the PR’s main change: adding neutral-IR encode/decode helpers for switchyard-translation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/switchyard-translation/src/helpers.rs`:
- Around line 146-159: Bound the pending SSE data in the streaming loop around
`buffer` and `sse::drain_next_sse_frame`: after draining all complete frames
from each chunk, reject the request when the remaining incomplete buffer exceeds
the configured maximum frame size. Add a multi-chunk test that omits the frame
delimiter and verifies the oversized pending frame is rejected.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: c3a2bb39-46ff-4a53-980b-a40929104739

📥 Commits

Reviewing files that changed from the base of the PR and between 93f0e3e and bb40d0f.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock, !Cargo.lock

📒 Files selected for processing (4)

• crates/switchyard-translation/Cargo.toml
• crates/switchyard-translation/src/helpers.rs
• crates/switchyard-translation/src/lib.rs
• crates/switchyard-translation/src/sse.rs

[nachiketb-nvidia]

nothing extra to add apart from Graham, just an additional pass checking if there are more idiomatic or standard ways of doing anything in this PR would be good