fix: URL-encode document names in document index client requests

[Michael-JB]

Description

Prior to this commit, we did not URL-encode document names when querying the document index. This caused problems when names have special characters, i.e., '/' or '?'. We do not need to URL-encode other parts of the URL (e.g., namespace or collection) as these are already constrained to URL-safe characters. In addition to this fix, this commit includes the following changes:

• update DocumentPath.from_slash_separated_str to correctly delimit paths whose document name contains a forward slash. Ideally we'd remove both this and DocumentPath.to_slash_separated_str in the future.
• parametrize tests to include special characters and increase coverage
• migrate to new document index collection to avoid CI conflicts

Before Merging

• [ ] Review the code changes
  • Unused print / comments / TODOs
  • Missing docstrings for functions that should have them
  • Consistent variable names
  • ...
• [x] Update changelog.md if necessary
• [ ] Commit messages should contain a semantic label and the ticket number
  • Consider squashing if this is not the case