* feat(gateway): paginate thread run history (#5282)
GET /api/threads/{thread_id}/runs stays a bare array of the newest 100
runs so LangGraph SDK clients keep working. Add GET /runs/page with a
(created_at, run_id) keyset cursor so callers can walk older history.
* fix(gateway): reject one-sided run history cursors
RunManager.list_by_thread now raises if only one of before_created_at
or before_run_id is set, matching the HTTP 422. Document the per-page
sort cost on the SQL keyset query, and add the missing CHANGELOG
[#5282] link definition.
* fix(gateway): round-trip run page cursors through query strings
Emit next_before_created_at with a Z suffix so '+' is not decoded as a
space. Accept that space, and Z, when parsing. Treat blank cursor
fields as absent and reject a non-ISO before_created_at in RunManager
so a harness caller cannot silently restart at the newest page.
* style(gateway): ruff-format run page cursor files
Collapse the one-sided cursor ValueError and the two before_created_at
asserts so ruff format --check passes at line-length 240.
