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

docs/reference/api/general.md

@@ -162,6 +162,12 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
     },
     "agent_stat_refresh_interval": 0,
     "ai": {
+      "aibridge_proxy": {
+        "cert_file": "string",
+        "enabled": true,
+        "key_file": "string",
+        "listen_addr": "string"
+      },
       "bridge": {
         "anthropic": {
           "base_url": "string",