fix(realtime): process every key in a presence diff's leaves

[hsusul]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

Fixes #1630

AsyncRealtimePresence._sync_diff uses break where it means continue:

for key, left_presences in leaves.items():
    current_presences = self.state.get(key, [])

    if len(current_presences) == 0:
        break

When a presence_diff carries leaves for a key the client isn't tracking, the loop aborts and every remaining key in that diff is dropped. Those presences stay in presence.state indefinitely and on_presence_leave never fires for them — a ghost user in the UI until the next full presence_state resync.

leaves comes straight off the wire in _on_diff_event, so the server controls both the contents and the key order; a diff listing an untracked key before a tracked one is enough. A client sees untracked keys routinely — it joined after that user left, or already removed the key in an earlier diff.

What is the new behavior?

continue, so only the untracked key is skipped. This matches Phoenix's presence.js, which this file is a port of and which returns from the per-key callback there.

_sync/presence.py is a stub (SyncRealtimePresence has no state handling), so there is no sync counterpart to update.

Tests

test_sync_diff_processes_every_leave_key in src/realtime/tests/test_presence.py — feeds a diff whose leaves list an untracked key before a tracked one, and asserts the tracked key is removed from state and its on_leave callback fires. It fails on main (bob survives the diff, no callback) and passes here.

Validation

• uv run --package realtime pytest src/realtime/tests/test_presence.py — 9 passed.
• uv run --package realtime pytest src/realtime/tests — 25 passed, 2 failed. The two failures are test_connection.py::test_postgrest_changes and ::test_postgrest_changes_on_different_tables; they fail identically on main without this change in my environment, and are unrelated to presence.
• uv run --package realtime mypy src/realtime/src/realtime src/realtime/tests — success, 20 source files.
• uv run ruff check / uv run ruff format --check — clean.
• git diff --check — clean.

The realtime suite runs against the local Supabase stack (make realtime.start-infra, which needs Docker and the Supabase CLI); I had it running, hence the live tests above passing.

Additional context

None.