Handle Non-Alphanumerical Edge Cases When Searching Themes By Colour

[chrstinalin]

Fixes: mozilla/addons#15022

Description

Moved all formatting to its own function (convert_to_hex) that either returns a valid 6-character hex string or raises a ValueError.

Context

Testing

Checklist

• [ ] Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• [ ] Successfully verified the change locally.
• [ ] The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• [ ] Add before and after screenshots (Only for changes that impact the UI).
• [ ] Add or update relevant docs reflecting the changes made.

[sentry[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: src/olympia/search/filters.py

Function	Unhandled Issue
get_es_query	TypeError: hsl() takes 3 positional arguments but 4 were given /api/v5/... Event Count: 5
get_es_query	TypeError: hsl() missing 3 required positional arguments: 'r', 'g', and 'b' /... Event Count: 1
get_es_query	TypeError: hsl() missing 1 required positional argument: 'b' /api/v5/ad... Event Count: 1

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