RFR: 8364263: HttpClient: Improve encapsulation of ProxyServer

[Volkan Yazici]

On Fri, 15 Aug 2025 13:44:31 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

>> Make `ProxyServer` not extend from `Thread`.
>
> Marked as reviewed by dfuchs (Reviewer).

@dfuch, @jaikiran, thanks so much for the reviews. I've verified that `tier2` passes.

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PR Comment: https://git.openjdk.org/jdk/pull/26786#issuecomment-3195582937