feat: enhance HAR provider, image handling, markdown upload & cache

- **g4f/Provider/har/__init__.py**
  - `get_models`/`create_async`: iterate over `(domain, harFile)` and filter with `domain in request_url`
  - `read_har_files` now yields `(domain, har_data)`; fixes file variable shadowing and uses `json.load`
  - remove stray `print`, add type hint for `find_str`, replace manual loops with `yield from`
  - small whitespace clean-up

- **g4f/Provider/needs_auth/Grok.py**
  - `ImagePreview` now passes `auth_result.cookies` and `auth_result.headers`

- **g4f/Provider/needs_auth/OpenaiChat.py**
  - add `Union` import; rename/refactor `get_generated_images` → `get_generated_image`
  - support `file-service://` and `sediment://` pointers; choose correct download URL
  - return `ImagePreview` or `ImageResponse` accordingly and stream each image part
  - propagate 422 errors, update prompt assignment and image handling paths

- **g4f/client/__init__.py**
  - drop unused `ignore_working` parameter in sync/async `Completions`
  - normalise `media` argument: accept single tuple, infer filename when missing, fix index loop
  - `Images.create_variation` updated to use the new media logic

- **g4f/gui/server/api.py**
  - expose `latest_version_cached` via `?cache=` query parameter

- **g4f/gui/server/backend_api.py**
  - optional Markdown extraction via `MarkItDown`; save rendered text as `<file>.md`
  - upload flow rewrites: copy to temp file, move to bucket/media dir, clean temp, store filenames
  - introduce `has_markitdown` guard and improved logging/exception handling

- **g4f/tools/files.py**
  - remove trailing spaces in HAR code-block header string

- **g4f/version.py**
  - add `latest_version_cached` `@cached_property` for memoised version lookup

g4f/gui/server/api.py

@@ -4,7 +4,7 @@ import logging
 import os
 import asyncio
 from typing import Iterator
-from flask import send_from_directory
+from flask import send_from_directory, request
 from inspect import signature
 
 from ...errors import VersionNotFoundError, MissingAuthError
@@ -87,7 +87,10 @@ class Api:
         latest_version = None
         try:
             current_version = version.utils.current_version
-            latest_version = version.utils.latest_version
+            if request.args.get("cache"):
+                latest_version = version.utils.latest_version_cached
+            else:
+                latest_version = version.utils.latest_version
         except VersionNotFoundError:
             pass
         return {