Reference the local image by its real filename in the e2e fixture

Drop the {{LOCAL_IMAGE}} placeholder in favour of a plain relative filename
(local_image.png), matching a real authoring workflow. The test renders from
the fixtures directory so the relative path resolves, as get_image expects.

Author: MattFisher

tests/substack/fixtures/full_features.md

@@ -57,7 +57,7 @@ plain code block without a language
 
 [![Linked image alt](https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png)](https://example.com/target)
 
-![A locally uploaded image]({{LOCAL_IMAGE}})
+![A locally uploaded image](local_image.png)
 
 `inline code with [^1] inside stays literal`
 

tests/substack/test_end_to_end.py

@@ -38,8 +38,6 @@
 TITLE = "python-substack e2e feature test"
 SUBTITLE = "Automated round-trip fixture"
 
-LOCAL_IMAGE_FILE = FIXTURES / "local_image.png"
-LOCAL_IMAGE_TOKEN = "{{LOCAL_IMAGE}}"
 LOCAL_IMAGE_ALT = "A locally uploaded image"
 
 
@@ -118,13 +116,16 @@ def walk(node):
 def _roundtrip(api: Api):
     """Post the fixture as a draft, read it back, and return its stored content."""
     markdown = MARKDOWN_FILE.read_text(encoding="utf-8")
-    # The local-image path is environment-specific, so it lives as a token in the
-    # fixture. We resolve it to a path relative to the current directory: get_image()
-    # uploads any path that os.path.exists(), and a relative path avoids the
-    # leading-slash handling that from_markdown applies to web-root paths.
-    markdown = markdown.replace(LOCAL_IMAGE_TOKEN, os.path.relpath(LOCAL_IMAGE_FILE))
     post = Post(TITLE, SUBTITLE, user_id=api.get_user_id())
-    post.from_markdown(markdown, api=api)
+    # Markdown image paths resolve relative to the working directory (this is how
+    # get_image uploads local files), so render from the fixtures directory just as
+    # a user authoring alongside their images would.
+    previous_cwd = os.getcwd()
+    os.chdir(FIXTURES)
+    try:
+        post.from_markdown(markdown, api=api)
+    finally:
+        os.chdir(previous_cwd)
     draft = api.post_draft(post.get_draft())
     draft_id = draft.get("id")
     try: