fix(targets) deduplicate nil-weight targets

[rainest]

What this PR does / why we need it:

Assume the Kong default 100 weight for targets that have nil weight when being deduplicated.

Which issue this PR fixes:

#5817 results in a segfault if deduplicated targets have no weight. I'm not sure if this happens in other cases, but it definitely happens for multi-replica Services that use the service-upstream annotation. We generate multiple targets all with the same Service hostname for service-upstream, which is maybe a bit silly, but I saw no reason to change it and wanted to make sure we cover any other situations that may generate nil-weight targets.

See segfault.txt for the fault. https://github.com/Kong/go-echo/blob/main/example_deploy.yaml will trigger it in current nightly.

Special notes for your reviewer:

I cannibalized TestIngressNamespaces() to test service-upstream in general. service-upstream is common enough (most mesh users, AFAIK) and we apparently never had an integration test that utilized it.

While looking for a test I could maybe slot it into, I discovered that a test added long long ago was no longer really testing anything in particular due to refactors elsewhere, so I have repurposed it into the service-upstream test.

The thing it was originally testing is mostly irrelevant, since we were mostly re-testing functionality built into controller-runtime.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR fix for unreleased change.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@b014561). Click here to learn what that means.

❗ Current head f169cfe differs from pull request most recent head f6eadd4. Consider uploading reports for the commit f6eadd4 to get more accurate results

Files	Patch %	Lines
...ternal/dataplane/translator/translate_upstreams.go	80.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             main   #5946   +/-   ##
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  Coverage        ?   67.3%           
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  Files           ?     179           
  Lines           ?   18304           
  Branches        ?       0           
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  Hits            ?   12327           
  Misses          ?    5032           
  Partials        ?     945           

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[randmonkey]

#5848
I see #5817 is backported to release/3.1.x and included in 3.1.3 and 3.1.4 releases.
I tried to deploy the services in https://github.com/Kong/go-echo/blob/main/example_deploy.yaml and an ingress pointing to 2 different port of the service and I did not reproduce the crash. @rainest can you test if it affects these versions, and provide a more detailed reproduce enviroment?

[rainest]

@randmonkey hmm, that's annoying, I guess there's no way to tell backports from the commit data.

The echo manifest lacks the ingress.kubernetes.io/service-upstream: "true" Service annotation necessary to trigger this. Not sure where I added that, but it's the missing piece. This does indeed occur on 3.1.x.

[team-k8s-bot]

The backport to release/3.1.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release/3.1.x release/3.1.x
# Navigate to the new working tree
cd .worktrees/backport-release/3.1.x
# Create a new branch
git switch --create backport-5946-to-release/3.1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 b8f7eb54420592f8ada2078082cb49170cf364e0
# Push it to GitHub
git push --set-upstream origin backport-5946-to-release/3.1.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.1.x

Then, create a pull request where the base branch is release/3.1.x and the compare/head branch is backport-5946-to-release/3.1.x.