fix: adapt image generation/editing to new Grok cardAttachment API

[JinchengGao-Infty]

Problem

Grok silently updated their web API, breaking both /v1/images/generations and /v1/images/edits:

1. Image generation via app-chat with explicit modelName/modelMode no longer triggers image generation — Grok returns text instead
2. Image results moved from modelResponse.generatedImageUrls to cardAttachment.jsonData.image_chunk format
3. Image editing uses a different card type (render_edited_image) than generation (render_generated_image)

Root Cause

Captured browser network requests via Playwright and compared with grok2api payloads:

Field	Browser (works)	grok2api (broken)
modelName	absent	"grok-3"
modelMode	absent	"MODEL_MODE_FAST"
modeId	"auto"	absent
toolOverrides	{"gmailSearch":false,...}	{}
Image data location	cardAttachment.image_chunk	looked for modelResponse.generatedImageUrls

Fix

app_chat.py: build_payload() supports model=None — omits modelName/modelMode, allowing modeId-based routing via request_overrides.

image.py: _stream_app_chat() and _collect_app_chat() now use model=None, mode=None with modeId: "auto".

image_edit.py:

• ImageStreamProcessor and ImageCollectProcessor parse new cardAttachment format with image_chunk.imageUrl (progress >= 100)
• Match both render_generated_image and render_edited_image card types
• Image URL constructed as https://assets.grok.com/{image_chunk.imageUrl}
• Edit path uses disableMemory=false, temporary=false and browser-matching toolOverrides

Grok cardAttachment Types

type	Purpose	Key field
render_generated_image	New image generation	image_chunk.imageUrl
render_edited_image	Image editing	image_chunk.imageUrl
render_searched_image	Web image search	image.original

Test Plan

• POST /v1/images/generations with grok-imagine-1.0 — returns image URL
• POST /v1/chat/completions with grok-imagine-1.0 (stream) — returns markdown image in chat chunk
• POST /v1/images/edits with grok-imagine-1.0-edit + image — returns edited image URL
• Normal POST /v1/chat/completions with grok-3 — text chat unaffected