refactor(core): replace websocket flags with a single transport preference

[rekram1-node]

Summary

Models had two WebSocket knobs that meant different things: capabilities.responsesWebsockets (a route fact precomputed by provider plugins, never user-settable) and websocket: boolean (user policy, model overrides provider, default true). Every other plugin-known field on Model.Info uses one pattern — plugin sets a default, user overrides, wrong overrides fail at request time — so this collapses WebSocket to the same shape.

• Drop Model.capabilities.responsesWebsockets. The route already knows whether it has a channel (open-responses-channel.ts falls back to HTTP when prepared.channel is absent; Azure's enabled(url) gate in @opencode/ai covers the URL/api-version checks the core plugin duplicated).
• Replace websocket?: boolean on Model.Info, Provider.Info, and their config entries with transport?: "http" | "websocket" (new Provider.Transport literal). Inherits model → provider like compaction.
• Remove the capability-setting loops from the OpenAI, xAI, and Azure plugins.

Request-time semantics

transport	Behavior
omitted	Offer the session executor; the route uses its channel when it has one, HTTP otherwise
"http"	Never offer the executor
"websocket"	Offer the executor; if the route has no channel, log a warning and fall back to HTTP

The warning is delivered via a new optional WebSocketChannelExecutor.unavailable: Effect<void> hook. HttpTransport.httpJson, the Open Responses channel transport's HTTP path, and the AI SDK adapter route in core/src/aisdk.ts all run it when handed an executor they cannot use, so every route type reports consistently. Core attaches it only when the user explicitly chose "websocket".

Notes

• The websocket config key landed in feat(core): enable the responses websocket by default #48140 and is not in any release tag, so no config-normalizer migration diagnostic is added.
• Regenerated packages/client types and packages/protocol/openapi.json.

Testing

• bun typecheck clean in schema, ai, core, server, protocol, client.
• ai: 1290 pass. schema: pass. core: 5374 pass; the 8 remaining failures are bash/shell-environment tests that fail identically on the base commit.
• Extended the Meta HTTP-only and Azure unsupported-endpoint tests to assert unavailable fires; updated the config inheritance and model-request policy tests to the three-way transport behavior.