Wildcard endpoint proxying + OpenRouter-style provider routing

[ericflo]

Summary

Replace the three hardcoded POST route handlers (/v1/chat/completions, /v1/messages, /v1/responses) with a single wildcard catch-all handler that proxies any HTTP method and path to workers. Adds OpenRouter-compatible provider routing preferences so clients can express provider ordering, inclusion/exclusion, and fallback behavior.

What changed

Protocol (modelrelay-protocol)

• RequestMessage gains method: String (defaults to "POST" for backward compat) and endpoint_path already existed
• RegisterMessage / RegisterAck gain endpoint_prefixes: Vec<String> so workers advertise which paths they handle
• New ProviderRouting struct with only, ignore, order, allow_fallbacks, sort fields

Server core (modelrelay-server/src/lib.rs)

• WorkerState tracks endpoint_prefixes and exposes supports_endpoint() for prefix matching
• submit_transport_request accepts endpoint_path, method, and Option<ProviderRouting>
• Worker selection respects endpoint capability and provider routing preferences
• find_eligible_worker_id_with_routing implements the full routing algorithm

HTTP layer (modelrelay-server/src/http.rs)

• Removed chat_completions_handler, messages_handler, responses_handler
• Added wildcard_handler as .fallback() — extracts method, full path, headers, body
• Model extracted from JSON body model field or X-Model header
• Provider routing from body provider field or X-Provider header (JSON)
• All HTTP methods supported (GET/POST/PUT/DELETE/PATCH/HEAD/OPTIONS)

Worker (modelrelay-worker)

• WorkerDaemonConfig gains endpoint_prefixes field
• CLI/env/config-file support for --endpoint-prefixes / ENDPOINT_PREFIXES
• forward_request uses dynamic method dispatch instead of hardcoded .post()

Backward compatibility

• All new protocol fields use #[serde(default)] / skip_serializing_if — existing workers/clients work unchanged
• method defaults to "POST", endpoint_prefixes defaults to empty (accept all)
• Provider routing is entirely optional

Test plan

• All 256 existing tests pass across protocol, server, worker, contract-tests, cloud, and web crates
• Manual test with worker registering endpoint prefixes
• Manual test with provider routing headers

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