feat(coderd): bill completed local tool batches in agent runtime - #28360

Stack: #28359 → #28360 (this PR) → #28361 → #28362

Summary

Completed local tool batches now count toward Coder Agent runtime (chat_messages.runtime_ms, the source of truth for hb_agent_runtime_v1), excluding sub-agent orchestration tools to avoid double counting, and without multiplying runtime for parallel tool calls.

Second of a four-PR stack splitting #28211. Stacked on #28359. Refs CODAGT-928 (https://linear.app/codercom/issue/CODAGT-928/track-local-tool-execution-for-agent-runtime).

Problem

Agent runtime previously measured only model invocation wall clock (stream open to fully consumed). Time spent executing local tools between steps, including file operations, terminal commands, workspace provisioning, and MCP tools, was deliberately excluded, which undercounts the product definition of "actively processing a task".

Naive inclusion has two hazards: a batch of parallel tool calls would bill N windows for one wall-clock wait, and wait_agent would re-bill child agents that already bill their own model and tool time.

Fix

Each completed local tool batch bills one window: the union of the billed tools' execution intervals, persisted as runtime_ms on a dedicated usage record appended after the batch's tool-result rows. The record is a tool-role message with visibility='model', so it never reaches the API, SSE, or clients, and prompt replay drops it because its single internal tool-batch-usage part converts to no provider content. Its content carries an audit payload (billed_ms, billed_calls) so the billed window is inspectable after the fact; real tool-result rows never carry batch-level runtime. Concurrent calls all start at batch start, so 5 parallel 10s reads bill 10s, not 50s; serial calls (SerialToolCalls) count only from their own launch, so unbilled waits before them do not count.

Sub-agent orchestration tools (spawn_agent, wait_agent, message_agent, interrupt_agent, list_agents, list_subagent_models, plus the deprecated close_agent alias) never extend the window: every chat, including children, applies the same rules to its own runtime, so a parent's wait_agent window would double count. A lone wait_agent bills 0 and appends no usage record; execute 10s in parallel with wait_agent 60s bills 10s. A test pins the unbilled set to the registered sub-agent tool catalog so they cannot drift.

GetTotalChatMessageRuntimeMsInRange already sums runtime_ms role-agnostically, so the usage record is picked up with no schema, query, or cron changes.

Client-executed dynamic tools, external agents, parked/idle time, and retry backoff remain unbilled. Interrupted batches still bill only the model window; the rest of the stack adds partial-window billing via the same usage record.

ExecuteLocalToolsOptions also gains an optional ToolBillingRecorder (RecordStart/RecordComplete) observing per-occurrence dispatch-order execution stamps. This PR wires no recorder; the next PR in the stack connects it to the message part buffer for interrupt billing.

NOTE: Reported agent runtime (hb_agent_runtime_v1) increases from deploy forward. There is no backfill and no feature flag.