fix(http): fix proxy memory leak and HTTPS proxy tunnel not triggered from env vars

[zcg]

Problem

When running zvm remote (or any command that fetches from a remote URL) with an HTTP proxy configured via environment variables (http_proxy, https_proxy), two issues occur:

1. Memory leak -- DebugAllocator reports leaked memory from createProxyFromEnvVar and initDefaultProxies in std.http.Client.
2. Fetch failure -- Failed to fetch version map because HTTPS requests go through the broken connectProxied() code path that returns HTTP 400.

Root Cause

Memory Leak

std.http.Client.deinit() does not free http_proxy / https_proxy objects -- the standard library comments them as "Pointer to externally-owned memory". The initDefaultProxies() function's API is designed for the caller to pass an ArenaAllocator whose lifetime the caller manages. However, the code was passing the raw DebugAllocator directly, so the proxy objects allocated on the heap were never freed.

Fetch Failure

The codebase has a workaround for connectProxied() returning HTTP 400: it manually establishes an HTTP CONNECT tunnel for HTTPS through proxies. But this workaround was gated on proxy_url.len > 0 -- i.e., only triggered when a proxy was explicitly configured via zvm proxy set. When the proxy was auto-detected from environment variables (the common case), the condition was false, and the request fell through to the broken client.fetch() -> connectProxied() path.

Fix

ArenaAllocator for proxy allocations

Create an ArenaAllocator (proxy_arena) in each function that creates an std.http.Client, pass its allocator to initClientProxy, and defer proxy_arena.deinit() to release all proxy memory when the function exits.

Expand CONNECT tunnel condition

Change the condition from:

if (protocol == .tls and proxy_url.len > 0)

to:

if (protocol == .tls)

and use resolveProxy() (which already checks both explicit URL and environment variables) to determine whether a proxy is available. This ensures the CONNECT tunnel workaround applies regardless of proxy source.

Affected Files

• src/network/http_client.zig -- downloadToFileWithProxy, downloadToMemoryWithProxy
• src/network/mirror_probe.zig -- probeThreadMainWindows (same pattern, Windows-only code path)

Testing

• zig build -- passes
• zig build test -- passes
• zig build run -- remote -- works correctly with http_proxy set, no memory leak errors, no fetch failures