perf(a2a): push ListTasks all-sessions query down to SQL

[QuentinBisson]

Follow-up to #2187, addressing #2197 (Copilot review finding).

What

collectUserTasks served an all-sessions ListTasks query by loading every session for the user, then every task per session, then filtering, sorting, and paginating in memory — an N+1 query whose result set and app-side memory scaled with the user's total task count even for a small page.

This replaces that path with a single ListUserTasks sqlc query that joins session to task, filters, orders by task.id, and paginates server-side, returning COUNT(*) OVER() as the total. The app now materializes one page instead of the user's entire task set.

This is a memory/transfer win, not (yet) a DB-latency win: as option 1 (no schema change) the query still scans the user's sessions and tasks — there is no index on session.user_id, and the status filter casts task.data::jsonb per row — so DB-side latency still scales with the user's task count. Option 2 (generated column + index) is the durable fix for that.

Status filtering (issue option 1)

status and statusTimestampAfter are filtered in SQL via a jsonb cast on task.data — no schema migration (option 1 from the issue, per the request to "start with option 1 for status for now").

Rows exist in two persisted shapes and both are matched:

shape	protocol_version	state string
v1	'v1'	TASK_STATE_WORKING
legacy	NULL	working

Both spellings are passed (status_v1, status_legacy) and matched with IN (...); the two vocabularies never overlap, so no row is silently dropped. data is always a JSON object (json.Marshal output) so ::jsonb never errors; the timestamp cast is guarded by a CASE (ordered evaluation) so a present-but-malformed value can't error the query.

Option 2 (generated column + index) remains the durable path for a later migration.

Behavior preserved

The single-session path keeps its fail-closed GetSession ownership check (a missing/other-user context is an error, not an empty page); the join scopes every path to the caller's own sessions. Ordering by task.id matches the prior in-memory sort, so page tokens stay stable.

Rebased onto the task-owner scoping from #2265, so the query now applies both gates the Go loop applied: the session belongs to the caller, and the task itself does under the task.user_id rule (including the NULL-owner fallback for pre-migration rows). The session join alone would have handed a foreign task parked in the caller's session to the caller.

COUNT(*) OVER() rides on the returned rows, so a page requested past the end of the set (e.g. rows deleted between paginated calls) comes back empty and would carry no total; CountUserTasks recovers the true filtered count in that case, matching the old in-memory behavior.

Tests

Adds a Postgres integration test (TestListUserTasks) covering both row shapes, cross-user isolation, the single-session predicate, status and timestamp filters, LIMIT/OFFSET pagination with a stable total, and the empty over-range page. Two cases cover the owner gates: a NULL-owner row resolving through its session, and a task owned by another user sitting in the caller's session staying hidden. Existing task_query_store unit tests pass unchanged against a fake that mirrors the query semantics.

[Copilot]

Pull request overview

This PR improves the scalability of A2A ListTasks for “all sessions” queries by pushing filtering, ordering, pagination, and total counting down into a single SQL query (removing the previous N+1 + in-memory aggregation path). It adds a dedicated ListUserTasks API/query that joins session→task, supports optional single-session scoping, and preserves stable ordering by task.id.

Changes:

• Add ListUserTasks + CountUserTasks sqlc queries to serve paginated task listings server-side (with COUNT(*) OVER() totals and a fallback count query for over-range pages).
• Update the A2A task query handler to call ListUserTasks instead of loading all sessions/tasks and filtering/sorting in memory.
• Add a Postgres integration test covering both persisted task row shapes, filters, pagination behavior, and cross-user isolation.

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
go/core/pkg/a2acompat/trpcv0/convert.go	Exposes a helper to map v1 task states to legacy persisted state strings.
go/core/internal/database/queries/tasks.sql	Adds ListUserTasks and CountUserTasks SQL queries with filtering/pagination and totals.
go/core/internal/database/gen/tasks.sql.go	Regenerates sqlc output for the new queries and parameter structs.
go/core/internal/database/gen/querier.go	Extends the generated Querier interface to include the new query methods.
go/core/internal/database/client_postgres.go	Implements ListUserTasks in the Postgres client (including total recovery when page is empty past end).
go/api/database/client.go	Introduces ListUserTasksParams and adds ListUserTasks to the DB client interface.
go/core/internal/a2a/task_query_store.go	Switches ListTasks implementation to call ListUserTasks and compute next page token from returned page size.
go/core/internal/a2a/task_query_store_test.go	Updates the in-memory fake store to mirror SQL semantics (filter/order/paginate + total).
go/core/internal/database/client_test.go	Adds a real-DB integration test for ListUserTasks behavior and edge cases.

Files not reviewed (2)

• go/core/internal/database/gen/querier.go: Generated file
• go/core/internal/database/gen/tasks.sql.go: Generated file

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