Refactor HaProxy exception handling

[jrhee17]

Motivation:

While working on #5793, I found that there were unnecessary ProxyConnectException related handling in the request handler. This is most likely due to the fact that HaProxyHandler works differently from the other proxy handlers in that the session is completed, and then a connection event is published.

This was probably when I was less familiar with netty. I propose that the HaProxyHandler complete the session similarly to the other handlers. A connection attempt is made first, and then the sessionPromise is succeeded or failed accordingly.

Modifications:

• HaProxyHandler doesn't succeed the sessionPromise immediately, but waits for the result of the connect future.
• Remove ProxyConnectException related logic in all places except for HttpSessionHandler#exceptionCaught

Result:

• The exception handling in AbstractHttpRequestHandler and HttpSessionHandler is a little simpler.

[trustin]

The simplification looks fine to me. I don't remember anymore why it is written that way anymore to be honest 😅

[github-actions]

🔍 Build Scan® (commit: 9f9226f)

Job name	Status	Build Scan®
build-windows-latest-jdk-21	✅	https://ge.armeria.dev/s/afcaua4bhikva
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/oltcme67nfolw
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/sdm6mfw5lqcwg
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/2zjnm5bnv4ttm
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/2yzi2tpgpzft4
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/6xg4bfjsoww5e
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/zr5l3pk6tbkfk
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/qgbturnzsxpmm

[minwoox]

Thanks! 👍 👍 👍

[ikhoon]

Thanks!