fix(chat): page agent history by compound cursor, not bare timestamp

[liqingb0220-stack]

Problem

In the agent 1:1 chat view, when a conversation contains a burst of messages that were persisted with an identical created_at (typically a batch of tool_call rows written in one turn) and that batch is larger than HISTORY_PAGE_SIZE (20), scrolling up never loads older messages. The tool count keeps inflating on every scroll (e.g. 23 → 35 → 155 …) and any message older than the batch — including the user's own first message — becomes unreachable. The data is intact; it just can never be paged to.

Root cause

The frontend derived its before pagination cursor from messages[0].created_at alone, dropping the |id component. The backend's before contract is <created_at>|<id> and it already emits a ready-to-use cursor field on every message (_base_message_entry).

For a timestamp-only cursor, _parse_message_cursor falls back to uuid.UUID(int=(1 << 128) - 1) (max UUID). The filter tuple_(created_at, id) < (before_created_at, MAX_UUID) is then true for every row sharing that timestamp, so each page re-returns the same equal-timestamp batch. messages[0].created_at of the next page is the same timestamp again → the cursor never advances below it. The backend comment already notes: "Legacy timestamp-only cursors may duplicate equal-timestamp messages … New clients should round-trip the emitted cursor."

Fix

Round-trip the backend cursor field at all four cursor-set sites in AgentDetailPage.tsx (refreshSessionMessages, session load, loadMoreHistoryMessages, loadMoreChatHistoryMessages), falling back to created_at only when cursor is absent. No backend change.

Verification

Reproduced on a real conversation with 23 same-timestamp tool_call rows:

• Before: scroll-up loops, tool count balloons to 155+, first user message never appears.
• After: scroll-up terminates with "已加载全部历史消息", tool count is the correct 23, and the first user message + agent greeting load correctly.

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