fix(openresponses): populate Content and accept bare {role,content} items (#10039) (#10040)

* fix(openresponses): populate Content and accept bare {role,content} items (#10039)

Fixes mudler/LocalAI#10039 — `/v1/responses` silently returned empty
output on any model whose YAML doesn't include a Go-side
`template.chat_message` block.

Three cooperating bugs:

* `convertORInputToMessages` populated only `StringContent` for string
  input and for the `input.Instructions` system message, leaving the
  `Content` (any) field nil.
* `TemplateMessages` gated all fallback content-rendering branches on
  `Content != nil && StringContent != ""` — but every branch in that
  function consumes `StringContent`, not `Content`. The `&&` silently
  dropped messages that had StringContent set and Content nil, producing
  an empty prompt that the 5× empty-retry guard then turned into a
  200 OK with `output: []`.
* The array-input branch of `convertORInputToMessages` dispatched on
  `itemMap["type"]` with no default, dropping bare `{role, content}`
  items emitted by the OpenAI Python SDK helper
  `client.responses.create(input=[{...}])`.

Fix:

* Set both `Content` and `StringContent` in the two openresponses
  message-construction sites that only set one.
* Treat a bare `{role, content}` item (no `type`) as
  `type: "message"` for OpenAI-SDK compatibility.
* Gate `TemplateMessages` fallback rendering on `StringContent != ""`,
  which is what every downstream branch in that function actually
  reads.

Regression test added to `evaluator_test.go` covering the fallback
path (no `ChatMessage` template) with a StringContent-only message,
both with and without a role mapping.

* test(openresponses): guard Content population and ToProto path (#10039)

Add regression tests for the two seams the original fix touched but
left uncovered:

* convertORInputToMessages must populate both Content and StringContent
  for plain string input and for bare {role, content} array items (the
  OpenAI SDK shape that omits the type discriminator). Both are
  functional reds against the pre-fix code.
* Messages.ToProto reads Content, not StringContent — this is the path
  UseTokenizerTemplate backends (imported GGUFs) take. The cases pin
  that contract so a future regression on the producer side is caught.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:claude-opus-4-7 [Claude Code]

---------

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