feat: back the per-chat cost endpoint with AI Gateway data - #27328

Stack Context

This stack removes native chat cost tracking and native chat usage limits, making the AI Gateway the single source of AI spend data and budget enforcement.

1. This PR: re-back the per-chat cost endpoint with AI Gateway data.
2. Remove native chat usage limits end to end, rewiring the sidebar indicator to gateway spend.
3. Remove native chat cost tracking end to end, deleting the Analytics/Spend cost UI.

What?

GET /api/experimental/chats/{chat}/cost summed chat_messages.total_cost_micros, which native chat cost tracking maintained. It now aggregates AI Gateway interception data instead, and has no native fallback.

• New GetAIBridgeChatCost query, authorized through the root chat so members can read their own chat's cost without gaining access to raw interception rows.
• Response fields renamed: priced_message_count -> request_count, unpriced_messages_having_usage_count -> unpriced_request_count.
• The chat summary sidebar keys its cost cache by root chat, and hides the cost row where the AI Gateway is off or unlicensed. The root cost is invalidated when a chat leaves an active status and when a generated title lands, since title generation bills its own gateway request.

GetChatModelUsageCostByChatID and the rest of native cost tracking are untouched here; PR 3 removes them.

Why?

Native cost tracking duplicates what the AI Gateway already records, and the two disagree. Repointing the endpoint first means the cost UI keeps working while the native implementation is deleted later in the stack.

Two behaviour changes follow from gateway semantics and are intentional:

• Requests, not messages. The gateway records interceptions, so counts are requests. Title-generation traffic now counts.
• Whole-tree totals. The gateway records the spawning chat's ID as the interception session ID, so a subagent's requests are attributed to its immediate parent, not always the root. Only a whole chat tree can be summed, so the query resolves the root and aggregates the tree, and every chat in a tree reports the same total. Native returned per-subtree totals.

Attribution and counting semantics

The aggregate groups token usage per interception before counting, so the reported numbers are per request even though a request records one usage row per provider response:

• RequestCount counts finished Coder Agents interceptions in the tree, including unpriced ones.
• UnpricedRequestCount counts requests with at least one usage row the gateway could not price. It is a subset of RequestCount.
• TotalCostMicros omits only unpriced usage, so a partially priced request still contributes its priced portion. The sidebar therefore says Excludes unpriced usage from N request(s) rather than claiming whole requests were dropped.

A recorded cost of zero is a free request, not an unpriced one. Usage without an effective group is excluded, matching what never reached ai_user_daily_spend.

Authorization

Reads go through ExtractChatParam plus ResourceChat, with no cost-specific RBAC widening. TestGetChatCost/MemberCanReadOwnChat covers a scoped agents-access member reading their own chat's cost, and MemberCannotReadOtherUsersChat still asserts 404 for a non-owner. Plain members without agents-access cannot create or read chats at all, so they never reach this endpoint.

Known limitation

AI Gateway data has its own retention period, 60 days by default and configured independently of chat retention, so spend for requests older than that is no longer reported. A chat whose gateway records have all been purged reports zero cost, which is indistinguishable from genuinely free usage under this contract. The endpoint documents the caveat; #27330 documents it on the Spend Management page.

In-flight interceptions are excluded, since cost is only known once the response is recorded. A chat's cost therefore lags the active turn by one request.

Rebase note

Rebased onto main after #27579 removed the ai-gateway-cost-control experiment. The per-chat cost row is now gated on the aibridge feature alone, matching how #27579 degated the other cost-control surfaces.

Mux prepared this PR on Mike's behalf.