refactor(accountManager): small fixes, optimizations and strict typing and

[solracsf]

Summary

Among others:

• Replaced error-prone json_last_error() checks with the JSON_THROW_ON_ERROR flag, explicitly catching JsonException to prevent silent serialization failures.
• Promoted all injected constructor dependencies and immutable class variables
• Replaced verbose anonymous closures
• Implemented null coalescing assignment operators

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI