fix: use min requested index for ecs rollback stage

[armistcxy]

What this PR does:

• Fixes buildPipelineStages in ECS plugin to reuse the smallest requested index for the appended rollback stage
• Adds unit tests covering the pipeline/quick-sync stage builders

Why we need it: sdk.PipelineStage.Index requires each returned index to be one of the indexes from the request. The previous implementation returned an invented index for ECS_ROLLBACK, so piped's validateStageIndexes rejected the plugin response

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[codecov]

Codecov Report

❌ Patch coverage is 52.94118% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.74%. Comparing base (8ed5a0e) to head (f3ff3fb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/pipedv1/plugin/ecs/deployment/plugin.go	0.00%	5 Missing ⚠️
pkg/app/pipedv1/plugin/ecs/deployment/pipeline.go	75.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7222      +/-   ##
==========================================
+ Coverage   29.68%   29.74%   +0.05%     
==========================================
  Files         601      601              
  Lines       64396    64408      +12     
==========================================
+ Hits        19116    19158      +42     
+ Misses      43789    43758      -31     
- Partials     1491     1492       +1     

Flag	Coverage Δ
.	23.59% <ø> (ø)
.-pkg-app-pipedv1-plugin-analysis	32.16% <ø> (ø)
.-pkg-app-pipedv1-plugin-ecs	33.64% <52.94%> (+1.90%)	⬆️
.-pkg-app-pipedv1-plugin-kubernetes	58.39% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	59.62% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	38.46% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	33.04% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	52.71% <ø> (ø)
.-pkg-plugin-sdk	49.85% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.51% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[Warashi]

We need this logic in all of the deployment plugins, so it's good to port it to the SDK?

[Copilot]

Pull request overview

This PR fixes the ECS pipedv1 plugin’s buildPipelineStages to ensure the appended ECS_ROLLBACK stage uses a request-provided stage index (specifically the minimum requested index), satisfying piped’s validateStageIndexes contract and preventing plugin responses from being rejected.

Changes:

• Update ECS pipeline stage builder to assign the rollback stage the minimum requested stage index (instead of an invented index).
• Add unit tests for ECS pipeline stage building, rollback index contract, and quick-sync stage building.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pkg/app/pipedv1/plugin/ecs/deployment/pipeline.go	Reuses the minimum requested index for ECS_ROLLBACK to satisfy piped’s stage-index validation.
pkg/app/pipedv1/plugin/ecs/deployment/pipeline_test.go	Adds unit tests for pipeline/quick-sync builders and a contract check ensuring returned indexes exist in the request.

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[armistcxy]

We need this logic in all of the deployment plugins, so it's good to port it to the SDK?

At this point I don't have my answer yet, but I think this part is actually repetitive among plugins. So let me investigate first

[Warashi]

We can open another PR to make this part of the SDK, so I approve this as-is.