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/exp_chats.go

@@ -1220,6 +1220,7 @@ func (api *API) postChats(rw http.ResponseWriter, r *http.Request) {
 		ClientType:         clientType,
 		SystemPrompt:       req.SystemPrompt,
 		InitialUserContent: contentBlocks,
+		APIKeyID:           apiKey.ID,
 		MCPServerIDs:       mcpServerIDs,
 		Labels:             labels,
 		DynamicTools:       dynamicToolsJSON,
@@ -3089,6 +3090,7 @@ func (api *API) postChatMessages(rw http.ResponseWriter, r *http.Request) {
 			CreatedBy:     apiKey.UserID,
 			Content:       contentBlocks,
 			ModelConfigID: modelConfigID,
+			APIKeyID:      apiKey.ID,
 			BusyBehavior:  busyBehavior,
 			PlanMode:      sendPlanMode,
 			MCPServerIDs:  req.MCPServerIDs,
@@ -3229,6 +3231,7 @@ func (api *API) patchChatMessage(rw http.ResponseWriter, r *http.Request) {
 		CreatedBy:       apiKey.UserID,
 		EditedMessageID: messageID,
 		Content:         contentBlocks,
+		APIKeyID:        apiKey.ID,
 		ModelConfigID:   editModelConfigID,
 	})
 	if editErr != nil {