feat(azure/savings-plans): wire SP rec collection into scheduler (closes #593)

[cristim]

Summary

• Adds savingsplans goroutine to the GetRecommendations fan-out in providers/azure/recommendations.go, mirroring the AWS SP slot in providers/aws/recommendations/client.go GetAllRecommendations
• The SP client's GetRecommendations intentionally returns an empty slice (Azure Benefits Recommendations API is still in preview); the goroutine slot is reserved so the scheduler picks up data the moment the upstream API stabilises, without needing a second wire-up PR
• Adds TestGetRecommendations_SavingsPlansServiceIncluded to pin that shouldIncludeService lets ServiceSavingsPlans through on both empty-params (all-services) and explicit-service sweeps

Test plan

• go build ./... passes
• go test github.com/LeanerCloud/CUDly/providers/azure/... passes (664 tests)
• New test TestGetRecommendations_SavingsPlansServiceIncluded covers empty-params, explicit-SP, and cross-service cases

Summary by CodeRabbit

Release Notes

• New Features
  • Added Azure Savings Plans to recommendation retrieval, expanding coverage to six Azure services (Compute, Database, Cache, CosmosDB, Savings Plans, and Advisor).

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📝 Walkthrough

Walkthrough

This PR extends Azure GetRecommendations to include Savings Plans as a sixth service in the multi-service fan-out pattern. A new goroutine conditionally fetches Savings Plans recommendations when requested, results are merged alongside existing services in deterministic order, and a table-driven test validates the service inclusion logic.

Changes

Savings Plans service integration

Layer / File(s)	Summary
Savings Plans fan-out implementation providers/azure/recommendations.go	Import Savings Plans client; extend concurrency variables to track spRecs/spErr; add conditional goroutine to fetch Savings Plans recommendations; include Savings Plans in deterministic result merge before Advisor; update method comment to document expanded canonical ordering (compute → database → cache → cosmosdb → savingsplans → advisor).
Savings Plans service inclusion test providers/azure/recommendations_test.go	New table-driven test TestGetRecommendations_SavingsPlansServiceIncluded asserts that shouldIncludeService includes ServiceSavingsPlans for empty params or explicit ServiceSavingsPlans selection, and excludes it when a different service is requested.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• LeanerCloud/CUDly#723: Updates the frontend Savings Plans detection and bucketing to recognize the "savingsplans" slug added by this backend fan-out extension.
• LeanerCloud/CUDly#592: Introduces the Azure Savings Plans service client and registers common.ServiceSavingsPlans in the provider factory, forming the infrastructure this PR extends.
• LeanerCloud/CUDly#259: Previous modification to GetRecommendations fan-out and deterministic result merging that established the multi-service concurrency pattern extended here.

Poem

A rabbit hops through recommendation lanes,
Adding Savings Plans to the six-service refrains,
Goroutines fan-out with deterministic grace,
Merging results in canonical place. 🐰💰

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat(azure/savings-plans): wire SP rec collection into scheduler' directly summarizes the main change: adding Savings Plans recommendation collection to the scheduler fan-out in Azure.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/593-wave5

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Verification note: providers/azure/services/savingsplans/client.go GetRecommendations currently returns a hardcoded empty slice (the Azure Benefits Recommendations API is in preview), so the new scheduler goroutine wires a no-op path - no Azure SP recommendation can reach the user yet, and the test only asserts shouldIncludeService(), not an end-to-end rec. That is fine as a reserved slot, but issue #593 asks for actual SP rec collection plus the commitmentopts SP probe equivalent. Recommend NOT auto-closing #593 on merge (scope this PR to 'reserve the goroutine slot' and keep #593 open for the real implementation when the preview API stabilizes), or split a follow-up issue. Leaving for a human to decide.

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