fix(sandbox): eliminate Box::leak memory leak in rewrite_forward_request

[johntmyers]

Summary

• Remove Box::leak in rewrite_forward_request that permanently leaked one String allocation per forward proxy request, causing unbounded RSS growth on long-running sandboxes
• Write the rewritten request line directly to the output buffer during iteration, eliminating the need to mutate a Vec<&str> element with a leaked &'static str

Related Issue

Closes #709

Changes

crates/openshell-sandbox/src/proxy.rs — rewrite_forward_request function:

• Removed the Box::leak(new_line.into_boxed_str()) block that mutated lines[0] with a permanently leaked allocation
• Made lines immutable (removed mut)
• Moved the request-line rewrite (METHOD absolute-uri HTTP/1.1 → METHOD path HTTP/1.1) into the output-building loop, writing method/path/version parts directly via extend_from_slice
• Added fallback else branch for malformed request lines (passes through unchanged)

Testing

• cargo check -p openshell-sandbox — clean
• cargo test -p openshell-sandbox -- proxy — 84/84 pass
• mise run pre-commit — all checks pass (pre-existing license header issue on unrelated file)

Checklist

• Change is scoped to the issue
• Existing tests cover the modified code path
• Pre-commit checks pass
• No secrets or credentials included