feat: route chatd provider traffic through aibridge (#25629)

## Summary

Routes chatd model calls backed by concrete AI Provider rows through the
in-process aibridge transport by default, with deployment options to use
direct provider routing when AI Gateway is disabled or chat AI Gateway
routing is disabled.

- Splits model routing into common, direct provider, and AI Gateway
paths behind a single deployment-mode entry point.
- Builds chatd models through explicit request, route, and options data.
Active API key attribution is passed explicitly instead of being hidden
inside generic model construction.
- For AI Gateway BYOK routes, resolves the user's provider key in chatd,
forwards it through provider-specific auth headers, and sets
`X-Coder-AI-Governance-Token` to the `delegated` marker so aibridge
preserves those headers while still stripping Coder-specific metadata.
- Keeps central provider credentials and deployment fallback credentials
out of forwarded provider auth headers, so AI Gateway central policy
remains authoritative.
- Redacts delegated provider auth from default string formatting to
avoid accidental plaintext logging of user BYOK credentials.
- Covers selected chat models, advisor overrides, title and quickgen
paths, subagent overrides, computer use model selection, and an
integration-style chat turn through the aibridge transport path.
- Persists initiating API key IDs on chat and queued user messages,
including subagent child messages, and fails closed for AI
Gateway-routed model builds without an active key.
- Removes unused `api_key_id` indexes while keeping the persistence
columns and foreign keys.
- Keeps the deployment option available through config and env parsing,
but hides it from CLI help and generated docs.
- Stabilizes the subagent poll fallback test so background CreateChat
processing cannot win the state transition under slower CI environments.

## Tests

- `go test ./coderd/x/chatd -run
'TestAIGatewayProviderAuthForUser|TestAIGatewayProviderAuthRedactsFormatting|TestResolveModelRouteForConfigAIGatewayProviderAuth|TestAIGatewayModelForwardsProviderAuth|TestProcessChat_AIGatewayRoutingUsesDelegatedAPIKey|TestAwaitSubagentCompletion'
-count=1`
- `go test ./coderd/aibridged -run
'TestServeHTTP_DelegatedAPIKey|TestServeHTTP_StripCoderToken' -count=1`
- `git diff --check HEAD~1..HEAD`
- `make lint`

> Mux working on behalf of Mike.

coderd/database/queries/chats.sql

@@ -763,6 +763,7 @@ WITH updated_chat AS (
 INSERT INTO chat_messages (
     chat_id,
     created_by,
+    api_key_id,
     model_config_id,
     role,
     content,
@@ -783,6 +784,7 @@ INSERT INTO chat_messages (
 SELECT
     @chat_id::uuid,
     NULLIF(UNNEST(@created_by::uuid[]), '00000000-0000-0000-0000-000000000000'::uuid),
+    NULLIF(UNNEST(@api_key_id::text[]), ''),
     NULLIF(UNNEST(@model_config_id::uuid[]), '00000000-0000-0000-0000-000000000000'::uuid),
     UNNEST(@role::chat_message_role[]),
     UNNEST(@content::text[])::jsonb,
@@ -1688,11 +1690,12 @@ RETURNING
     *;
 
 -- name: InsertChatQueuedMessage :one
-INSERT INTO chat_queued_messages (chat_id, content, model_config_id)
+INSERT INTO chat_queued_messages (chat_id, content, model_config_id, api_key_id)
 VALUES (
     @chat_id,
     @content,
-    sqlc.narg('model_config_id')::uuid
+    sqlc.narg('model_config_id')::uuid,
+    sqlc.narg('api_key_id')::text
 )
 RETURNING *;