feat: add upstream proxy support to aiproxy for passthrough requests (#21512)

## Description

Adds upstream proxy support for AI Bridge Proxy passthrough requests.
This allows aiproxy to forward non-allowlisted requests through an
upstream proxy. Currently, the only supported configuration is when
aiproxy is the first proxy in the chain (client → aiproxy → upstream
proxy).

## Changes

* Add `--aibridge-proxy-upstream` option to configure an upstream
HTTP/HTTPS proxy URL for passthrough requests
* Add `--aibridge-proxy-upstream-ca` option to trust custom CA
certificates for HTTPS upstream proxies
* Passthrough requests (non-allowlisted domains) are forwarded through
the upstream proxy
* MITM'd requests (allowlisted domains) continue to go directly to
aibridge, not through the upstream proxy
* Add tests for upstream proxy configuration and request routing

Closes: https://github.com/coder/internal/issues/1204

cli/testdata/coder_server_--help.golden

@@ -165,6 +165,17 @@ AI BRIDGE PROXY OPTIONS:
       --aibridge-proxy-listen-addr string, $CODER_AIBRIDGE_PROXY_LISTEN_ADDR (default: :8888)
           The address the AI Bridge Proxy will listen on.
 
+      --aibridge-proxy-upstream string, $CODER_AIBRIDGE_PROXY_UPSTREAM
+          URL of an upstream HTTP proxy to chain passthrough (non-allowlisted)
+          requests through. Format: http://[user:pass@]host:port or
+          https://[user:pass@]host:port.
+
+      --aibridge-proxy-upstream-ca string, $CODER_AIBRIDGE_PROXY_UPSTREAM_CA
+          Path to a PEM-encoded CA certificate to trust for the upstream proxy's
+          TLS connection. Only needed for HTTPS upstream proxies with
+          certificates not trusted by the system. If not provided, the system
+          certificate pool is used.
+
 CLIENT OPTIONS: 
 These options change the behavior of how clients interact with the Coder.
 Clients include the Coder CLI, Coder Desktop, IDE extensions, and the web UI.

cli/testdata/server-config.yaml.golden

@@ -798,6 +798,15 @@ aibridgeproxy:
   domain_allowlist:
     - api.anthropic.com
     - api.openai.com
+  # URL of an upstream HTTP proxy to chain passthrough (non-allowlisted) requests
+  # through. Format: http://[user:pass@]host:port or https://[user:pass@]host:port.
+  # (default: <unset>, type: string)
+  upstream_proxy: ""
+  # Path to a PEM-encoded CA certificate to trust for the upstream proxy's TLS
+  # connection. Only needed for HTTPS upstream proxies with certificates not trusted
+  # by the system. If not provided, the system certificate pool is used.
+  # (default: <unset>, type: string)
+  upstream_proxy_ca: ""
 # Configure data retention policies for various database tables. Retention
 # policies automatically purge old data to reduce database size and improve
 # performance. Setting a retention duration to 0 disables automatic purging for