feat!: default CODER_AI_GATEWAY_ENABLED to true (#25575)

`CODER_AI_GATEWAY_ENABLED` / `CODER_AIBRIDGE_ENABLED` is now being defaulted to `true` now that it will be used by Coder Agents.

If you previously had this value disabled explicitly, that value will persist.

cli/testdata/coder_server_--help.golden

@@ -157,7 +157,7 @@ AI GATEWAY OPTIONS:
           Length of time to retain data such as interceptions and all related
           records (token, prompt, tool use).
 
-      --ai-gateway-enabled bool, $CODER_AI_GATEWAY_ENABLED (default: false)
+      --ai-gateway-enabled bool, $CODER_AI_GATEWAY_ENABLED (default: true)
           Whether to start an in-memory AI Gateway instance.
 
       --ai-gateway-max-concurrency int, $CODER_AI_GATEWAY_MAX_CONCURRENCY (default: 0)

cli/testdata/server-config.yaml.golden

@@ -768,8 +768,8 @@ chat:
 aibridge:
   # Deprecated: use --ai-gateway-enabled or CODER_AI_GATEWAY_ENABLED instead.
   # Whether to start an in-memory aibridged instance.
-  # (default: false, type: bool)
-  enabled: false
+  # (default: true, type: bool)
+  enabled: true
   # Deprecated: use --ai-gateway-openai-base-url or CODER_AI_GATEWAY_OPENAI_BASE_URL
   # instead. The base URL of the OpenAI API.
   # (default: https://api.openai.com/v1/, type: string)
@@ -867,8 +867,8 @@ aibridge:
   circuit_breaker_max_requests: 3
 ai_gateway:
   # Whether to start an in-memory AI Gateway instance.
-  # (default: false, type: bool)
-  enabled: false
+  # (default: true, type: bool)
+  enabled: true
   # The base URL of the OpenAI API.
   # (default: https://api.openai.com/v1/, type: string)
   openai_base_url: https://api.openai.com/v1/