fix(proxy): do not inject no_proxy/NO_PROXY when no proxy is configured

[zoutao212]

问题

managed_proxy_env_pairs() 在 global_proxy_enabled == false 时，仍然追加了本机直连白名单
（no_proxy / NO_PROXY）。结果是：每一次"受管启动"都给子进程塞了一份"手动启动"永远不会有的环境变量。

对 Antigravity IDE 而言，这会打断渲染进程与本机 Language Server 之间的通信通道：

• 渲染进程日志出现 [unknown] Failed to fetch: ConnectError
• Language Server 收不到任何对话请求（streamGenerateContent == 0）
• 切换账户后的首个会话，Agent 面板会静默丢弃消息
  （用户只能关闭再重新打开 IDE 才能恢复）

复现

同一账户、同一状态数据库，唯一变量是启动方式：

启动方式	是否注入 no_proxy	渲染进程 Failed-to-fetch	LS streamGenerateContent
手动启动	否	0	2
手动启动	否	0	2
手动启动	是	1	0
受管启动（本工具）	是	1	0

在启动同一个 IDE 二进制之前仅注入 no_proxy / NO_PROXY，就足以复现该故障；
清除这两个变量后故障即消失。--reuse-window 参数与 Chromium net-log 抓包这两个变量，
均已被同一组对照实验排除。

修复

只有在"确实存在代理"时才输出代理相关环境变量：

• 已配置受管代理，或
• 当前进程本身已经带有代理变量（此时本机直连白名单仍然有益）

除此之外直接返回空列表，使受管启动与手动启动的环境逐位一致。

验证

在 upstream/main（deacbe44）之上执行 cargo check -p cockpit-tools --lib 通过。

说明：本机无法运行 cargo test —— lib 单元测试可执行文件启动即中止，
报 STATUS_ENTRYPOINT_NOT_FOUND（缺少 comctl32 v6 manifest）。
因此本次未添加单元测试。

Problem

managed_proxy_env_pairs() appends the loopback bypass list (no_proxy / NO_PROXY)
even when global_proxy_enabled == false. As a result, every managed launch passes the
child an environment that a manually launched app never has.

For Antigravity IDE this breaks the channel between the renderer and the local
language server:

• renderer logs [unknown] Failed to fetch: ConnectError
• the language server receives zero chat requests (streamGenerateContent == 0)
• the Agent panel silently discards the first message after an account switch
  (the user must close and reopen the IDE to recover)

Reproduction

Same account, same state DB — only the launch path differs:

launch path	no_proxy injected	renderer Failed-to-fetch	LS streamGenerateContent
manual	no	0	2
manual	no	0	2
manual	yes	1	0
managed (this tool)	yes	1	0

Injecting only no_proxy / NO_PROXY before starting the same IDE binary is
enough to reproduce it; clearing them makes it disappear. The --reuse-window flag
and Chromium net-log capture were both ruled out by the same experiment matrix.

Fix

Emit proxy variables only when a proxy is actually in play:

• a managed proxy is configured, or
• the current process already carries a proxy variable (then the loopback bypass
  list is still useful)

Otherwise return an empty list, so a managed launch is byte-for-byte identical
to a manual launch.

Verification

cargo check -p cockpit-tools --lib on top of upstream/main (deacbe44) passes.

Note: cargo test cannot run in the authoring environment — the lib test binary
aborts immediately with STATUS_ENTRYPOINT_NOT_FOUND (missing comctl32 v6 manifest).
No unit test is added for that reason.