TLS ECH: Update ECH configuration parsing logic to be more robust

[jamesliu96]

Key Changes: Robust Configuration String Splitting

1. Swapped out aggressive, un-delimited splitting in favor of strings.SplitN(c.EchConfigList, "+", 2).
   This ensures that if the downstream DoH or h2c URL contains query parameters or search paths that inherently use the plus sign (+) (e.g., https://some-dns.com/dns-query?token=123+xyz), it will no longer be erroneously truncated.

2. IPv6 and port detection support. Improved the port-appending mechanism for fallback classic UDP servers using standard library net.SplitHostPort.
   The previous strings.Contains(udpServerAddr, ":") check was fundamentally flawed when users inputted explicit IPv6 formats like udp://[2001:db8::1], bypassing the default :53 port injection.

[RPRX]

@Fangliding ready 时说一声

[Fangliding]

就这样吧

[RPRX]

@jamesliu96 顺便说一下你这个 commit 邮箱挺怪的，是 jamesliu96@users.noreply.github.com，前面少了 2211002+

[jamesliu96]

@RPRX Thanks for the heads-up! According to GitHub's documentation (see GitHub Docs), accounts created before July 18, 2017 use the USERNAME@users.noreply.github.com format without the ID prefix. That's why it looks a bit different.