refactor(server): serve libsy algorithms without profiles

[nachiketb-nvidia]

What

Replace the components-v2 profile server with a thin HTTP host for libsy algorithms.

• ServerState maps route model IDs directly to Arc<dyn Algorithm> values.
• Repeated --route MODEL=TARGET[,TARGET...] arguments create independent libsy Random
  algorithms over one shared TranslatingLlmClient.
• OpenAI Chat Completions, OpenAI Responses, and Anthropic Messages requests are decoded, run
  through the selected algorithm, and encoded back to the caller's format.
• Harness headers are normalized by switchyard-protocol::Metadata::from_headers and retained for
  upstream forwarding.
• The components-v2 config loader, profile registry, Rust stats endpoints, PyO3 server binding, and
  Python wrapper for the Rust server are removed.
• switchyard serve --config remains on the existing Python FastAPI path instead of dispatching
  profile-only configs to Rust.
• A concise crate README documents the current CLI.

Provider-specific Responses tool-turn translation is intentionally split into #111.

Why

The Rust server should own HTTP transport and algorithm dispatch while libsy owns routing behavior.
Keeping YAML parsing, profile construction, and profile-specific state in the server duplicated the
old architecture and prevented the server from being a small host for arbitrary libsy algorithms.

How

Each inbound request selects an algorithm by its model, translates the wire payload into a libsy
request, calls Algorithm::run, and converts the buffered or streaming result directly into an Axum
response. The CLI currently builds uniform-random algorithms only; all configured targets share one
upstream base URL, API key, and wire format.

switchyard-server \
  --route switchyard/general=model/a,model/b \
  --route switchyard/coding=model/c,model/d \
  --base-url https://example.com/v1

The server retains canonical HTTP errors, routing headers, /v1/models, /health, TLS, streaming,
and --dry-run. The removed Rust stats endpoints depended on the deleted profile runtime and are
not part of the libsy server surface.

What to review

1. Direct model-to-algorithm lookup and rejection of empty or duplicate route IDs.
2. Independent random target sets sharing one translating LLM client.
3. Protocol-owned metadata normalization and preservation of forwarded HTTP headers.
4. Inbound decode -> Algorithm::run -> direct buffered/SSE response conversion.
5. Streaming termination, upstream error mapping, and routing response headers.
6. Removal of profile/config ownership and the Rust server's Python bridge.
7. Consolidated tests: endpoint behavior remains covered while libsy owns Random correctness.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p switchyard-server (7 passed)
• git diff --check origin/main

#79 is merged and this branch is based directly on main.

[github-actions]

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[coderabbitai]

Walkthrough

The pull request replaces the Rust profile-config server with a libsy algorithm-based HTTP server, routes profile-config serving through Python, removes obsolete bindings, and updates Responses translation for signed reasoning and tool-call arguments.

Changes

Libsy HTTP server

Layer / File(s)	Summary
CLI and runtime construction crates/switchyard-server/Cargo.toml, crates/switchyard-server/src/cli.rs, crates/switchyard-server/README.md	The CLI accepts repeatable routes, upstream settings, and wire-format options, then builds routed algorithms and runtime configuration.
State, routing, and HTTP handlers crates/switchyard-server/src/lib.rs	Server state, unified LLM endpoints, model listing, TLS handling, errors, lifecycle, and startup output now use libsy algorithms.
Response encoding and SSE termination crates/switchyard-server/src/response.rs, crates/switchyard-server/src/sse.rs	Responses use libsy encoders, and failed SSE streams stop without emitting later events or [DONE].
HTTP integration coverage crates/switchyard-server/tests/server.rs	Tests cover routing, formats, random targets, streaming, health/models, missing endpoints, and errors.

Responses translation contracts

Layer / File(s)	Summary
Buffered Responses input encoding crates/switchyard-translation/src/codecs/responses/buffered.rs, crates/switchyard-translation/tests/request_translation.rs	Signed reasoning is omitted from Responses input, while tool-call arguments are serialized as JSON strings and covered by translation tests.

Python profile-serving path

Layer / File(s)	Summary
Remove Rust profile-server bindings crates/switchyard-py/Cargo.toml, crates/switchyard-py/src/lib.rs, switchyard_rust/core.py	The native profile-server dependency, registration, and protocol declaration are removed.
Use the Python HTTP application switchyard/cli/switchyard_cli.py, switchyard/lib/profiles/loader.py, switchyard/lib/processors/random_routing_request_processor.py	The profile-config command uses the Python application directly and removes obsolete profile inspection and deprecation paths.
Serving documentation and test updates docs/cli_reference.md, docs/internal/metrics_reference.md, tests/test_profile_migration.py, tests/test_route_bundle.py, tests/test_serve_profile_config.py	Documentation and tests are updated for Python-based profile serving and no longer verify Rust delegation.

Estimated code review effort: 4 (Complex) | ~60 minutes

Poem

A rabbit hops through routes anew,
With libsy paths and headers blue.
Signed thoughts fade from the stream,
Tool-call strings now neatly gleam.
Python serves the profiles bright—
SSE stops at errors’ night.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 43.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: the server now serves libsy algorithms instead of profile-based configs.

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