[CASCL-1386] (2/11) Build the evict-legacy-nodes execution plan

[L3n41c]

What does this PR do?

Implements BuildPlan: computes which node groups (targets) to drain from the cluster snapshot, filtering out Datadog-managed entities.

Motivation

PR #3026 is too large to review as a single change. This is part 2 of 11 of a stack that splits it into small, individually-reviewable pieces that each build and pass tests on their own. See #3026 for the full feature context and end-to-end QA.

Additional Notes

Stacked PR — the base branch is lenaic/CASCL-1386-evict-01-skeleton, not main, so the diff shows only this step's change. Implements logic the skeleton (part 1) left stubbed. The code is taken verbatim from #3026 (rebased onto main); there are no logic changes versus #3026.

Minimum Agent Versions

N/A — kubectl-datadog plugin only.

Describe your test plan

go test ./cmd/kubectl-datadog/autoscaling/cluster/... passes at this commit. Full end-to-end QA is tracked on #3026.

Checklist

• PR has at least one valid label
• qa/skip-qa label applied (not independently shippable; QA tracked on [CASCL-1386] Add evict-legacy-nodes subcommand to drain non-Datadog node groups #3026)
• All commits are signed

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.98246% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.90%. Comparing base (a97a49e) to head (67c1c0b).

Files with missing lines	Patch %	Lines
.../kubectl-datadog/autoscaling/cluster/evict/plan.go	92.98%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           lenaic/CASCL-1386-evict-01-skeleton    #3161      +/-   ##
=======================================================================
+ Coverage                                43.74%   43.90%   +0.16%     
=======================================================================
  Files                                      388      388              
  Lines                                    30950    31005      +55     
=======================================================================
+ Hits                                     13538    13614      +76     
+ Misses                                   16524    16501      -23     
- Partials                                   888      890       +2     

Flag	Coverage Δ
unittests	43.90% <92.98%> (+0.16%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../kubectl-datadog/autoscaling/cluster/evict/plan.go	95.00% <92.98%> (+95.00%)	⬆️

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