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

---

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

coderd/database/dump.sql

@@ -1625,7 +1625,8 @@ CREATE TABLE aibridge_tool_usages (
     invocation_error text,
     metadata jsonb,
     created_at timestamp with time zone NOT NULL,
-    provider_tool_call_id text
+    provider_tool_call_id text,
+    provider_item_id text
 );
 
 COMMENT ON TABLE aibridge_tool_usages IS 'Audit log of tool calls in intercepted requests in AI Bridge';
@@ -1638,6 +1639,8 @@ COMMENT ON COLUMN aibridge_tool_usages.injected IS 'Whether this tool was inject
 
 COMMENT ON COLUMN aibridge_tool_usages.invocation_error IS 'Only injected tools are invoked.';
 
+COMMENT ON COLUMN aibridge_tool_usages.provider_item_id IS 'Specific to the OpenAI Responses API: the unique id of the output item that carried the tool call. Distinct from provider_tool_call_id (the call_id correlation key), which is empty for hosted tools. Empty for the chat completions and Anthropic messages APIs, which have no separate item id.';
+
 CREATE TABLE aibridge_user_prompts (
     id uuid NOT NULL,
     interception_id uuid NOT NULL,