feat: add Relay routing Decision API

[bbednarski9]

What

Summary

Adds the first, independently functional layer of the NeMo Relay integration:

• a versioned JSON routing-request and routing-decision contract;
• decision-only Random and LLM routing through the existing profile runtimes;
• POST /v1/routing/decision with distinct malformed, unknown-profile, and unsupported-profile errors;
• Relay-compatible route metadata, including the configured Switchyard target ID as backend_id.

This is the Decision API slice of a previous end-to-end implementation, split into a focused public-repository change.

Why

Relay needs Switchyard to select a configured target without dispatching that target itself. Keeping this contract and the two existing v2 routing profiles in a focused first PR makes the wire API, profile capability boundary, and server error behavior reviewable before ATOF state or Cascade routing are introduced.

How tested

Testing summary

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p switchyard-components-v2 (46 unit tests plus profile integration suites)
• cargo test -p switchyard-server (21 server tests plus migration coverage)
• cargo test --workspace --exclude switchyard-py
• cargo check -p switchyard-py
• Loopback classifier integration verifies LLM classification without calling the selected target.
• uv run ruff check . — not applicable; no Python source changed.
• uv run mypy switchyard — not applicable; no Python source changed.
• uv run pytest tests/ — not applicable; the affected API is covered by Rust component/server tests.

Checklist

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

Notes for reviewers

Review guidelines

Suggested review order:

1. decision.rs for the wire contract, protocol/materialization validation, and response conversion.
2. profile.rs and registry.rs for the single-runtime capability path.
3. Random/LLM profile adapters to verify that decision-only routing stops before selected-target dispatch.
4. Server endpoint/error tests for Relay compatibility and status-code precedence.

The legacy Relay payload still includes decision_profile.router. Serde intentionally ignores that additive field; profile_id is authoritative. The response still includes router and all five route fields required by Relay.

Previous feedback addressed

• Replaced the parallel DecisionRuntimeRegistry design with Profile::decide() on the same Arc<dyn Profile> already used for serving; configured runtimes are built once.
• Removed typed request-side DecisionProfile.router while retaining legacy wire compatibility for the extra field.
• Validates schema, attempt bounds, and inbound protocol before profile lookup, so malformed requests remain 400 even when the profile ID is unknown.
• Rejects unknown inbound profiles rather than silently treating them as OpenAI Chat.
• Validates actual user prompt material before an LLM classifier call, including adversarial empty/assistant-only shapes.
• Replaced fallible test .expect() usage with typed error propagation.
• Keeps malformed (400), unknown profile (404), and unsupported profile (422) failures distinct.

ATOF ingestion and Relay snapshots are intentionally deferred to #9; Relay-aware Cascade routing and unified session metadata are deferred to #11.

Follow-up fixes

• Keeps Random and LLM decision-response conversion with their profile implementations; decision.rs now owns only shared contract and materialization helpers.
• Validates required identity/protocol strings and rejects contradictory materialization modes or missing current_request.body before profile execution.
• Adds focused envelope-validation coverage.

[sabhatinas]

@nachiketb-nvidia for viz