Added tests for all proxy related code in the overmind CLI. (#2992)

## Fix AWS SDK v2 Proxy Configuration and Add Comprehensive Proxy Tests

### Problem

We needed to assess whether or not we've supported proxies in the CLI in
order to make sure that we can use this at one of our customers.

### Audit Summary

An audit of HTTP clients found one issue (now fixed). All other clients
already respect proxy settings:
- ✅ `otelhttp.DefaultClient` (uses `http.DefaultTransport`)
- ✅ AWS SDK with Terraform provider config (explicitly configured)
- ✅ GCP SDK, Kubernetes client-go, WebSocket dialers (all use transports
that respect proxy)
- ✅ ⚠️ AWS SDK v2 default config (now fixed)

### Solution

**1. Fixed AWS SDK v2 Default Configuration** (`cli/cmd/explore.go`)
- Configured AWS SDK v2 HTTP client to use `http.ProxyFromEnvironment`
when falling back to default credentials
- Ensures consistent proxy behavior across all AWS SDK usage scenarios

**2. Added Comprehensive Proxy Tests** (`cli/cmd/auth_client_test.go`)
- Created test infrastructure with a mock HTTP proxy server
- Added tests for:
  - `newRetryableHTTPClient()` proxy configuration
- `AuthenticatedChangesClient` proxy preservation through transport
chain
  - WebSocket dialer proxy support (HTTP CONNECT)
- Tests use `localhost.df.overmind-demo.com` (resolves to 127.0.0.1) to
avoid proxy bypass

### Verification

All tests pass:
- `TestNewRetryableHTTPClientRespectsProxy`
- `TestAuthenticatedChangesClientUsesProxy`
- `TestWebSocketDialerUsesProxy`

GitOrigin-RevId: f10d00e85ee193028923885e491936bec02203c9

Author: tphoney