fix(sliding-sync): drop stale invite state for joined rooms in sidebar cache

[eleboucher]

Description

On homeservers that keep sending invite_state for a room the user has already joined (e.g. Continuwuity), accepting a local invite left the room "half-joined": after a restart it reappeared as a pending invite. The sidebar cache persisted the stale invite_state, and matrix-js-sdk treats invite_state as authoritative (it short-circuits to invite membership and ignores required_state), so hydration kept resurrecting the invite.

Fixes #1283

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

code review and autocomplete