fix(openapi/upload): more edge case handling for URL uploads

[kanadgupta]

🚥 Resolves CX-1793

🧰 Changes

expanding on the work from #1344, this PR tweaks a bunch of logic / DX improvements to better account for the following rdme openapi upload edge cases:

• URL uploads that lack a path but have a custom slug with a JSON file extension (e.g., rdme openapi upload https://example.com --custom-slug.json) will now properly read that file and set that slug
• URL uploads that lack a path but have a custom slug with a YAML file extension (e.g., rdme openapi upload https://example.com --custom-slug.yml) will now properly read that file and set that slug
• URL uploads that lack a path and there's no --slug passed (e.g., rdme openapi upload https://example.com) will now default to openapi.json and emit a warning about this behavior
• URL uploads with a path that lacks a file extension and there's no --slug passed (e.g., rdme openapi upload https://example.com/some-path) will now properly handle JSON/YAML and set the slug to that path (e.g., some-path.json). [applicable for both creates and updates]

🧬 QA & Testing

honestly the more test cases i added, the more rework i realized this logic required 😮‍💨 the fact that they're passing brings me a lot of comfort here lol

[kanadgupta]

the diff in this file is pretty rough apologies 😭 i had to shuffle a bit of logic around to hit all of these edge cases and tried to simplify logic and add comments wherever i could. might be easier to just read the file top-to-bottom to be honest 😮‍💨

[kanadgupta]

other than the new tests, the other test refactors are either stricter assertions or minor variable renaming!