feat: add core AI MITM proxy daemon (#21296)

## Description

Adds the core AI Bridge MITM proxy daemon. This proxy intercepts HTTPS traffic, decrypts it using a configured CA certificate, and forwards requests to AIBridge for processing.

## Changes

* Added `aibridgeproxyd` package with the core proxy server implementation
* Added configuration options: `CODER_AIBRIDGE_PROXY_ENABLED`, `CODER_AIBRIDGE_PROXY_LISTEN_ADDR`, `CODER_AIBRIDGE_PROXY_CERT_FILE`, `CODER_AIBRIDGE_PROXY_KEY_FILE`
* Added tests for server initialization and MITM functionality

Closes https://github.com/coder/internal/issues/1180

coderd/apidoc/docs.go

@@ -12009,6 +12009,23 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.AIBridgeProxyConfig": {
+            "type": "object",
+            "properties": {
+                "cert_file": {
+                    "type": "string"
+                },
+                "enabled": {
+                    "type": "boolean"
+                },
+                "key_file": {
+                    "type": "string"
+                },
+                "listen_addr": {
+                    "type": "string"
+                }
+            }
+        },
         "codersdk.AIBridgeTokenUsage": {
             "type": "object",
             "properties": {
@@ -12108,6 +12125,9 @@ const docTemplate = `{
         "codersdk.AIConfig": {
             "type": "object",
             "properties": {
+                "aibridge_proxy": {
+                    "$ref": "#/definitions/codersdk.AIBridgeProxyConfig"
+                },
                 "bridge": {
                     "$ref": "#/definitions/codersdk.AIBridgeConfig"
                 }

coderd/apidoc/swagger.json

@@ -10673,6 +10673,23 @@
 				}
 			}
 		},
+		"codersdk.AIBridgeProxyConfig": {
+			"type": "object",
+			"properties": {
+				"cert_file": {
+					"type": "string"
+				},
+				"enabled": {
+					"type": "boolean"
+				},
+				"key_file": {
+					"type": "string"
+				},
+				"listen_addr": {
+					"type": "string"
+				}
+			}
+		},
 		"codersdk.AIBridgeTokenUsage": {
 			"type": "object",
 			"properties": {
@@ -10772,6 +10789,9 @@
 		"codersdk.AIConfig": {
 			"type": "object",
 			"properties": {
+				"aibridge_proxy": {
+					"$ref": "#/definitions/codersdk.AIBridgeProxyConfig"
+				},
 				"bridge": {
 					"$ref": "#/definitions/codersdk.AIBridgeConfig"
 				}