feat: byok-observability for aibridge (#23808)

## Summary

Adds `credential_kind` and `credential_hint` columns to
`aibridge_interceptions` to record how each LLM request was
authenticated and provide a masked credential identifier for audit
purposes.

This enables admins to distinguish between centralized API keys,
personal API keys, and subscription-based credentials in the
interceptions audit log.

## Changes

- New migration adding `credential_kind`and `credential_hint` to
`aibridge_interceptions`
- Updated `InsertAIBridgeInterception` query and proto definition to
carry the new fields
- Wired proto fields through `translator.go` and `aibridgedserver.go` to
the database

Depends on https://github.com/coder/aibridge/pull/239

coderd/database/migrations/000464_aibridge_credential_kind.down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE aibridge_interceptions
+    DROP COLUMN IF EXISTS credential_kind,
+    DROP COLUMN IF EXISTS credential_hint;
+
+DROP TYPE IF EXISTS credential_kind;

coderd/database/migrations/000464_aibridge_credential_kind.up.sql

@@ -0,0 +1,12 @@
+CREATE TYPE credential_kind AS ENUM ('centralized', 'byok');
+
+-- Records how each LLM request was authenticated and a masked credential
+-- identifier for audit purposes. Existing rows default to 'centralized'
+-- with an empty hint since we cannot retroactively determine their values.
+ALTER TABLE aibridge_interceptions
+    ADD COLUMN credential_kind credential_kind NOT NULL DEFAULT 'centralized',
+    -- Length capped as a safety measure to ensure only masked values are stored.
+    ADD COLUMN credential_hint CHARACTER VARYING(15) NOT NULL DEFAULT '';
+
+COMMENT ON COLUMN aibridge_interceptions.credential_kind IS 'How the request was authenticated: centralized or byok.';
+COMMENT ON COLUMN aibridge_interceptions.credential_hint IS 'Masked credential identifier for audit (e.g. sk-a***efgh).';