feat(api-server): inline image inputs on /v1/chat/completions and /v1/responses (#12969)

OpenAI-compatible clients (Open WebUI, LobeChat, etc.) can now send vision
requests to the API server. Both endpoints accept the canonical OpenAI
multimodal shape:

  Chat Completions: {type: text|image_url, image_url: {url, detail?}}
  Responses:        {type: input_text|input_image, image_url: <str>, detail?}

The server validates and converts both into a single internal shape that the
existing agent pipeline already handles (Anthropic adapter converts,
OpenAI-wire providers pass through). Remote http(s) URLs and data:image/*
URLs are supported.

Uploaded files (file, input_file, file_id) and non-image data: URLs are
rejected with 400 unsupported_content_type.

Changes:

- gateway/platforms/api_server.py
  - _normalize_multimodal_content(): validates + normalizes both Chat and
    Responses content shapes. Returns a plain string for text-only content
    (preserves prompt-cache behavior on existing callers) or a canonical
    [{type:text|image_url,...}] list when images are present.
  - _content_has_visible_payload(): replaces the bare truthy check so a
    user turn with only an image no longer rejects as 'No user message'.
  - _handle_chat_completions and _handle_responses both call the new helper
    for user/assistant content; system messages continue to flatten to text.
  - Codex conversation_history, input[], and inline history paths all share
    the same validator. No duplicated normalizers.

- run_agent.py
  - _summarize_user_message_for_log(): produces a short string summary
    ('[1 image] describe this') from list content for logging, spinner
    previews, and trajectory writes. Fixes AttributeError when list
    user_message hit user_message[:80] + '...' / .replace().
  - _chat_content_to_responses_parts(): module-level helper that converts
    chat-style multimodal content to Responses 'input_text'/'input_image'
    parts. Used in _chat_messages_to_responses_input for Codex routing.
  - _preflight_codex_input_items() now validates and passes through list
    content parts for user/assistant messages instead of stringifying.

- tests/gateway/test_api_server_multimodal.py (new, 38 tests)
  - Unit coverage for _normalize_multimodal_content, including both part
    formats, data URL gating, and all reject paths.
  - Real aiohttp HTTP integration on /v1/chat/completions and /v1/responses
    verifying multimodal payloads reach _run_agent intact.
  - 400 coverage for file / input_file / non-image data URL.

- tests/run_agent/test_run_agent_multimodal_prologue.py (new)
  - Regression coverage for the prologue no-crash contract.
  - _chat_content_to_responses_parts round-trip coverage.

- website/docs/user-guide/features/api-server.md
  - Inline image examples for both endpoints.
  - Updated Limitations: files still unsupported, images now supported.

Validated live against openrouter/anthropic/claude-opus-4.6:
  POST /v1/chat/completions  → 200, vision-accurate description
  POST /v1/responses         → 200, same image, clean output_text
  POST /v1/chat/completions [file] → 400 unsupported_content_type
  POST /v1/responses [input_file]  → 400 unsupported_content_type
  POST /v1/responses [non-image data URL] → 400 unsupported_content_type

Closes #5621, #8253, #4046, #6632.

Co-authored-by: Paul Bergeron <paul@gamma.app>
Co-authored-by: zhangxicen <zhangxicen@example.com>
Co-authored-by: Manuel Schipper <manuelschipper@users.noreply.github.com>
Co-authored-by: pradeep7127 <pradeep7127@users.noreply.github.com>

website/docs/user-guide/features/api-server.md

@@ -83,6 +83,25 @@ Standard OpenAI Chat Completions format. Stateless — the full conversation is
 }
 ```
 
+**Inline image input:** user messages may send `content` as an array of `text` and `image_url` parts. Both remote `http(s)` URLs and `data:image/...` URLs are supported:
+
+```json
+{
+  "model": "hermes-agent",
+  "messages": [
+    {
+      "role": "user",
+      "content": [
+        {"type": "text", "text": "What is in this image?"},
+        {"type": "image_url", "image_url": {"url": "https://example.com/cat.png", "detail": "high"}}
+      ]
+    }
+  ]
+}
+```
+
+Uploaded files (`file` / `input_file` / `file_id`) and non-image `data:` URLs return `400 unsupported_content_type`.
+
 **Streaming** (`"stream": true`): Returns Server-Sent Events (SSE) with token-by-token response chunks. For **Chat Completions**, the stream uses standard `chat.completion.chunk` events plus Hermes' custom `hermes.tool.progress` event for tool-start UX. For **Responses**, the stream uses OpenAI Responses event types such as `response.created`, `response.output_text.delta`, `response.output_item.added`, `response.output_item.done`, and `response.completed`.
 
 **Tool progress in streams**:
@@ -119,6 +138,25 @@ OpenAI Responses API format. Supports server-side conversation state via `previo
 }
 ```
 
+**Inline image input:** `input[].content` can contain `input_text` and `input_image` parts. Both remote URLs and `data:image/...` URLs are supported:
+
+```json
+{
+  "model": "hermes-agent",
+  "input": [
+    {
+      "role": "user",
+      "content": [
+        {"type": "input_text", "text": "Describe this screenshot."},
+        {"type": "input_image", "image_url": "data:image/png;base64,iVBORw0K..."}
+      ]
+    }
+  ]
+}
+```
+
+Uploaded files (`input_file` / `file_id`) and non-image `data:` URLs return `400 unsupported_content_type`.
+
 #### Multi-turn with previous_response_id
 
 Chain responses to maintain full context (including tool calls) across turns:
@@ -330,7 +368,7 @@ In Open WebUI, add each as a separate connection. The model dropdown shows `alic
 ## Limitations
 
 - **Response storage** — stored responses (for `previous_response_id`) are persisted in SQLite and survive gateway restarts. Max 100 stored responses (LRU eviction).
-- **No file upload** — vision/document analysis via uploaded files is not yet supported through the API.
+- **No file upload** — inline images are supported on both `/v1/chat/completions` and `/v1/responses`, but uploaded files (`file`, `input_file`, `file_id`) and non-image document inputs are not supported through the API.
 - **Model field is cosmetic** — the `model` field in requests is accepted but the actual LLM model used is configured server-side in config.yaml.
 
 ## Proxy Mode