Fix protocol handling bug in ContentFetcher #91

Summary
The githubhttps:// protocol is mishandled in ContentFetcher.fetch_content, causing the unstripped full URL to be passed to _fetch_github_https_file which expects owner/repo/branch/path.

Details

• File: struct_module/content_fetcher.py
• Current logic special-cases content_location.startswith("http") and bypasses prefix stripping, so "githubhttps://..." is forwarded incorrectly.

Proposed fix

• Match and handle explicit prefixes in deterministic order before any generic http/https branch.
• Only treat raw "https://" as an HTTP URL; for "githubhttps://", strip the prefix and route to _fetch_github_https_file with owner/repo/branch/path.
• Add unit tests covering github://, githubhttps://, githubssh://, and https:// to avoid regressions.

Acceptance criteria

• githubhttps://owner/repo/branch/path fetches the expected content without errors.
• Existing protocols (file://, github://, githubssh://, https://) continue to work.
• Tests added passing in CI.