### What this PR does

Before this PR:
- NewApi image generation hardcoded `response_format: 'b64_json'`,
forcing all responses to base64 regardless of model capability
- All providers that handle base64 responses stored `urls:
validFiles.map(f => f.name)` — just filenames — causing `handleRetry` to
fail with invalid URLs
- Chat-generated base64 images were stored as full
`data:image/...;base64,...` URIs inside `ImageMessageBlock.url`, which
got included in subsequent message history and caused HTTP 413 errors
(#12602)

After this PR:
- NewApi no longer forces `response_format: 'b64_json'`; the API
defaults to returning URLs, with base64 as a transparent fallback
- Base64 responses in NewApi, Aihubmix, and Ovms now correctly store
`urls: []` (no retry URL exists for inline base64 data)
- Chat-generated base64 images are persisted to disk immediately; the
block stores a `file` reference and a local `file://` URL instead of the
raw data URI
- `ImageGenerationMiddleware` now prefers `block.file` over `block.url`
when collecting images from previous assistant messages

https://linux.do/t/topic/1681973/18
Fixes #12602

### Why we need it and why it was done in this way

The following tradeoffs were made:
- Removing `response_format: 'b64_json'` from NewApi follows the same
pattern as Aihubmix's `gpt-image-1` handling — no explicit format is
set, and the response handler already supports both URL and base64
paths.
- Storing `urls: []` for base64 responses is semantically correct: there
is no remote URL to retry from once the base64 payload is gone.
- Persisting base64 chat images to disk is the minimal fix for #12602;
it reuses the existing `saveBase64Image` IPC path without changing any
message schema or Redux state shape.

The following alternatives were considered:
- Keeping `response_format: 'b64_json'` and fixing only the `urls` field
— rejected because NewApi supports URL responses natively, and URL mode
is more efficient.

### Breaking changes

None.

### Special notes for your reviewer

The `imageCallbacks.ts` change means that after a base64 image is
generated in chat, the `ImageMessageBlock` will have `file` set and
`url` as a local `file://` path. Existing messages already stored with
raw base64 URLs are unaffected (they will continue to display correctly
via the existing `parseDataUrl` path in `messageConverter.ts`).

### Checklist

- [x] PR: The PR description is expressive enough and will help future
contributors
- [x] Code: Write code that humans can understand and Keep it simple
- [x] Refactor: You have left the code cleaner than you found it (Boy
Scout Rule)
- [ ] Upgrade: Impact of this change on upgrade flows was considered and
addressed if required
- [x] Documentation: Not required — no user-facing feature added, only
bug fixes
- [x] Self-review: I have reviewed my own code before requesting review
from others

### Release note

```release-note
Fix base64 image handling in paintings (NewApi, Aihubmix, Ovms): images now return as URLs by default, preventing invalid retry URLs. Fix chat image generation storing large base64 data URIs in message history, which caused HTTP 413 errors on follow-up messages.
```

---------

Signed-off-by: Pleasurecruise <[email protected]>
Signed-off-by: suyao <[email protected]>
Co-authored-by: suyao <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>