feat: add TLS listener support to aibridgeproxyd (#22411)

## Description

Adds optional TLS support for the AI Bridge Proxy listener. When TLS cert and key files are provided, the proxy serves over HTTPS instead of plain HTTP.

## Changes

* New configuration options to enable TLS on the proxy listener 
* Wraps the TCP listener in `tls.NewListener` when configured
* Tests for validation errors, invalid files, and full integration (tunneled + MITM) through a TLS listener

Note: Documentation for TLS listener setup and client configuration will be handled in a follow-up PR.
Related to: https://github.com/coder/internal/issues/1335

coderd/apidoc/docs.go

@@ -12576,6 +12576,12 @@ const docTemplate = `{
                 "listen_addr": {
                     "type": "string"
                 },
+                "tls_cert_file": {
+                    "type": "string"
+                },
+                "tls_key_file": {
+                    "type": "string"
+                },
                 "upstream_proxy": {
                     "type": "string"
                 },

coderd/apidoc/swagger.json

@@ -11184,6 +11184,12 @@
 				"listen_addr": {
 					"type": "string"
 				},
+				"tls_cert_file": {
+					"type": "string"
+				},
+				"tls_key_file": {
+					"type": "string"
+				},
 				"upstream_proxy": {
 					"type": "string"
 				},