fix(core): URL-encode bgColor parameter in mermaid.ink API calls (#34444)
The background_color parameter was not being URL-encoded when building
the mermaid.ink API URL. Named colors like 'white' get prefixed with '!'
which is a special character that must be URL-encoded as '%21' to avoid
HTTP 400 errors from the mermaid.ink API.
Changes:
- Import urllib.parse for URL encoding
- URL-encode the background_color value using urllib.parse.quote()
- Add unit tests for both named colors and hex colors
Fixes #34444