fix(auth): remove legacy session credential fallbacks and improve code clarity

[joshtrichards]

• Resolves: #na

Summary

• Removes no longer needed legacy fallback (used only for v19->v20; was added in Fix undefined class property access after upgrade from 19 to 20 #22641).
• Explicitly handles JSON decode failures.
• Includes minor readability improvements, such as splitting out session fallback (non-legacy) into a dedicated function.
• Updates docblocks.
• Adds clearer exception messages

Outside of dropping the no longer applicable v19 legacy fallback behavior, there should be no breaking changes here. Other changes are purely code readability (e.g. re-ordering, separation of concerns, adjusting logic to not need temporary flag variable, minimize nesting, short/focused docblocks).

TODO

• Have to finish adjusting the unit tests

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)