fix(publish): keep the server path prefix in upload URLs

[hunger]

pixi publish --target-channel artifactory://host/artifactory/repo uploaded to https://host/repo/<subdir>/<pkg> and got a 404: the channel was taken from the last path segment and the rest of the path was then cleared with set_path(""). JFrog Artifactory serves repositories under an /artifactory prefix, so that prefix is part of the server URL, not something to discard.

Keep every segment before the channel as the server's base path, with a trailing slash so rattler_upload's Url::join appends rather than replaces, and share the split between the artifactory, quetz and prefix backends, which all had the same lossy logic. Credential lookup is unaffected: rattler_networking matches non-S3 URLs on the host alone.

The three 'missing channel name' errors now show the expected URL shape, and the docs and CLI help spell out that a path prefix belongs in the URL.

Fixes #6622

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Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.