feat(perps): support account notifications in session reads and events

[kartojal]

Description

Adds perps account notifications to the SDK, covering both REST reads and live session events.

What's included

• Notification models (models/perps/notifications.py): typed, discriminated union PerpsNotification covering position opened/increased/reduced/closed, limit order canceled, liquidation warning (isolated and cross variants, discriminated by margin_type), and position liquidated. Entries carry account-scoped read state (read_at) via PerpsNotificationEntry.
• Session reads:
  • session.list_notifications(since_seq=..., limit=...) newest-first paginator whose pages also report the account's unread count and durable_source_seq high-water mark (for backfilling after a gap).
  • session.mark_notifications_read(ids=... | up_to=...) mark by explicit ids, or everything up to and including a given entry.
• Session events: new PerpsNotificationEvent on the notifications channel, added to PerpsSessionEvent.
• Resync handling: notification frames carry the source engine sequence, which is not dense per channel, so local sequence-gap detection is disabled for notifications. Instead, server-sent resync control frames are surfaced as PerpsResyncEvent with a new reason="server" (plus optional timestamp), where sequence is the highest engine sequence among the dropped notifications.

Tests

Unit coverage for the account actions (listing, paging with since_seq, mark-read), event parsing (including server resync frames), and session wiring (~380 new test lines).

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Note

Medium Risk
New authenticated account APIs and session channel behavior; list_fills pagination semantics change may affect existing callers relying on the old cursor scheme.

Overview
Adds perps account notifications end-to-end: discriminated notification models, REST list (newest-first pages with unread and durable_source_seq) and mark read (by ids or inclusive up_to), plus session list_notifications / mark_notifications_read.

Live session work subscribes to notifications, emits PerpsNotificationEvent, maps server resync frames to PerpsResyncEvent (reason="server"), and skips client sequence-gap resync on that channel so sparse engine sequences do not false-trigger gaps.

list_fills switches from client-side descending history cursors to the API’s native sort, cursor, and trade-id pagination (optional initial_cursor).

Public exports include notification paginator types and PerpsSortDirection.

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