feat: record provider_item_id for tool usage (#26856)

## Summary

Plumbs the Responses output item id (added as `ToolUsageRecord.ItemID` in #26855) through to the database, captured independently of the `provider_tool_call_id` correlation key. Hosted tools (`web_search_call`, etc.) only have an item id; agentic tools have both.

`provider_item_id` is specific to the OpenAI Responses API; it stays empty for chat completions and Anthropic messages, which have no separate item id.

## Changes

- Migration `000534`: nullable `provider_item_id` column on `aibridge_tool_usages`.
- Proto: `item_id` field 11 on `RecordToolUsageRequest`.
- Server handler: persists `provider_item_id` and adds it to structured logging.
- Translator: maps `ToolUsageRecord.ItemID` to the proto field.

## Tests

- `TestRecordToolUsageProviderItemID`: real-database round-trip asserting `provider_item_id` persists for both hosted and agentic tools, independently of `provider_tool_call_id`.

Stacked on #26855. Linear: AIGOV-96

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_This PR was produced by opencode (agent) using the_ _`anthropic/claude-opus-4-8`_ _model, under human direction and review._

coderd/database/dbgen/dbgen.go

@@ -2078,6 +2078,7 @@ func AIBridgeToolUsage(t testing.TB, db database.Store, seed database.InsertAIBr
 		InterceptionID:     takeFirst(seed.InterceptionID, uuid.New()),
 		ProviderResponseID: takeFirst(seed.ProviderResponseID, "provider_response_id"),
 		ProviderToolCallID: takeFirst(seed.ProviderToolCallID),
+		ProviderItemID:     takeFirst(seed.ProviderItemID),
 		Tool:               takeFirst(seed.Tool, "tool"),
 		ServerUrl:          serverURL,
 		Input:              takeFirst(seed.Input, "input"),