Refactor FXIOS-15399 [Native Error Pages] NativeErrorPageHelper review nits

[ananyashreesharma]

📜 Tickets

JIRA: FXIOS-15399 (follow-up to certificate exception UI work, code-only nits)
GitHub: Related to #33011

💡 Description

Addresses post-merge review nits on NativeErrorPageHelper after #31933:

• Expand FXIOS-14569 TODO with the Jira ticket title for context.
• Mark nested Constants as private.
• Move host/description heuristic substrings into Constants.

Note: This PR is a part of Outreachy

🧩 Implementation

• firefox-ios/Client/Frontend/NativeErrorPage/NativeErrorPageHelper.swift

📝 Checklist

• I filled in the ticket numbers and a description of my work
• I updated the PR name to follow our PR naming guidelines
• I ensured unit tests pass and considered adding tests for new code
• If working on UI, I checked and implemented accessibility
• If adding telemetry, I read the data stewardship requirements and requested a data review
• If adding or modifying strings, I read the guidelines and requested a string review from l10n
• If needed, I updated documentation and added comments to complex code

🔗 Related
Follow-up to #31933