fix(streaming): comply with OpenAI usage / stream_options spec (#9815)

* fix(streaming): comply with OpenAI usage / stream_options spec (#8546)

LocalAI emitted `"usage":{"prompt_tokens":0,...}` on every streamed
chunk because `OpenAIResponse.Usage` was a value type without
`omitempty`. The official OpenAI Node SDK and its consumers
(continuedev/continue, Kilo Code, Roo Code, Zed, IntelliJ Continue)
filter on a truthy `result.usage` to detect the trailing usage chunk;
LocalAI's zero-but-non-null usage on every intermediate chunk made
that filter swallow every content chunk and surface an empty chat
response while the server log looked successful.

Changes:

- `core/schema/openai.go`: `Usage *OpenAIUsage \`json:"usage,omitempty"\``
  so intermediate chunks no longer carry a `usage` key. Add
  `OpenAIRequest.StreamOptions` with `include_usage` to mirror OpenAI's
  request field.
- `core/http/endpoints/openai/chat.go` and `completion.go`: keep using
  the `Usage` struct field as an in-process channel for the running
  cumulative, but strip it before JSON marshalling. When the request
  set `stream_options.include_usage: true`, emit a dedicated trailing
  chunk with `"choices": []` and the populated usage (matching the
  OpenAI spec and llama.cpp's server behavior).
- `chat_emit.go`: new `streamUsageTrailerJSON` helper; drop the
  `usage` parameter from `buildNoActionFinalChunks` since chunks no
  longer carry usage.
- Update `image.go`, `inpainting.go`, `edit.go` to wrap their Usage
  values with `&` for the new pointer field.
- UI: send `stream_options:{include_usage:true}` from the React
  (`useChat.js`) and legacy (`static/chat.js`) chat clients so the
  token-count badge keeps populating now that the server is
  spec-compliant.

Tests:

- New `chat_stream_usage_test.go` pins the spec invariants:
  intermediate chunks have no `usage` key, the trailer JSON has
  `"choices":[]` and a populated `usage`, and `OpenAIRequest` parses
  `stream_options.include_usage`.
- Update `chat_emit_test.go` to reflect that finals no longer embed
  usage.

Verified against the live LocalAI instance: before the fix Continue's
filter logic swallowed 16/16 token chunks; with the new shape it
yields 4/5 and routes usage through the dedicated trailer chunk.

Fixes #8546

Assisted-by: Claude:opus-4.7 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* fix(streaming): silence errcheck on usage trailer Fprintf

The new spec-compliant `stream_options.include_usage` trailer writes
were flagged by errcheck since they're new code (golangci-lint runs
new-from-merge-base on master); the surrounding `fmt.Fprintf` data:
writes are grandfathered. Drop the return values explicitly to match
the linter's contract without adding a nolint shim.

Assisted-by: Claude:opus-4.7 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

---------

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>