fix: expand tilde (~) in vision_analyze local file paths (#2585)

Path('~/.hermes/image.png').is_file() returns False because Path
doesn't expand tilde. This caused the tool to fall through to URL
validation, which also failed, producing a confusing error:
'Invalid image source. Provide an HTTP/HTTPS URL or a valid local
file path.'

Fix: use os.path.expanduser() before constructing the Path object.
Added two tests for tilde expansion (success and nonexistent file).

tools/vision_tools.py

@@ -242,7 +242,7 @@ async def vision_analyze_tool(
         logger.info("User prompt: %s", user_prompt[:100])
         
         # Determine if this is a local file path or a remote URL
-        local_path = Path(image_url)
+        local_path = Path(os.path.expanduser(image_url))
         if local_path.is_file():
             # Local file path (e.g. from platform image cache) -- skip download
             logger.info("Using local image file: %s", image_url)