feat(commitmentopts): probe Savings Plans offerings live (closes #108)

[cristim]

Summary

• Adds SavingsPlansProber that calls DescribeSavingsPlansOfferings once per plan type (Compute, EC2Instance, SageMaker, Database) and emits per-product service keys aligned with SERVICE_FIELDS in settings.ts
• Reuses walkPaginated, normalizePayment, and durationToTerm from the existing RI probe infrastructure
• Introduces dedupeRaw to collapse duplicate (duration, payment) pairs that arise because the SP API returns one row per ProductType (EC2/Lambda/Fargate) within a plan type
• Registers SavingsPlansProber in DefaultProbers() alongside the six RI probers
• Empty plan-type results are silently dropped so the frontend falls back to hardcoded rules rather than treating an empty DB row as a probe sentinel

Test plan

• go test ./internal/commitmentopts/... passes (66 tests including 8 new SP prober tests)
• go test ./internal/api/... -short passes (1151 tests)
• New tests cover: standard combos, all four plan types, empty-result drop, error propagation, page cap, dedup across ProductType variants, DefaultProbers registration

Closes #108

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between bc7bf0f and cb903e4.

📒 Files selected for processing (3)

• go.mod
• internal/commitmentopts/probe.go
• internal/commitmentopts/probe_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/issue-108-sp-probe

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