fix(aws): fail loud on unknown payment-option/term/engine -- remove silent defaults #1084

Problem

Several AWS provider converter functions silently substitute fabricated values when given unrecognized input, then proceed to query offerings and (on the purchase path) commit money to the wrong product:

H3 `recommendations/converters.go` `convertPaymentOption` silently returns `PaymentOptionNoUpfront` for any unrecognized payment option string, causing the SP recommendation-fetch path to query CE for the wrong payment option and return recs with wrong savings/upfront/recurring figures.

H4 `services/elasticache/client.go` `convertPaymentOption` silently returns `"Partial Upfront"` for any unknown option and proceeds to `PurchaseReservedCacheNodesOffering` on that option. The RDS sibling already errors correctly; this is an inconsistency as well as a money bug.

M4 `recommendations/parser_services.go` `parseRDSDetails` defaults `AZConfig` to `"single-az"` when CE omits `DeploymentOption`. Single-AZ and multi-AZ RDS RIs have different prices and do not cover each other; the wrong AZConfig causes `findOfferingID` to query for the wrong RI class.

M5 `recommendations/parser_services.go` `resolveEC2Tenancy` collapses any unrecognized tenancy value (including `"host"` for Dedicated Hosts, which has no EC2 RI product) to `"default"`, risking purchase of a default-tenancy RI for a Dedicated Host workload.

M6 `services/rds/client.go` `normalizeEngineName` hardcodes edition guesses: any Oracle engine -> `oracle-se2`, any SQL Server -> `sqlserver-se`, unknown Aurora variant -> `aurora-mysql`. Oracle/SQL Server editions (SE2/EE, SE/EE/Web/Express) are different products at different prices.

L1/L2 `recommendations/converters.go` `convertTermInYears` and `convertLookbackPeriod` silently default to 1-year / 7-day on unrecognized input, mis-fetching recommendations for the wrong term/lookback window.

Also includes H1/H2 savingsplans fail-loud fixes (cherry-picked from the now-merged #1039 branch): `convertPaymentOption` and `convertTermToSeconds` in `savingsplans/client.go` no longer default to All Upfront / 1yr on unknown input.

M1/M2/M3 `services/ec2/client.go`: `getOfferingClass` raw `"convertible"` string literals replaced with `types.OfferingClassTypeConvertible`; `buildEC2OfferingQuery` errors on empty Platform (purchase path); raw `"Linux/UNIX"` literals centralized into `defaultEC2Platform` constant.

Fix

• Fail loud (return error) on all unrecognized/absent input on money-affecting paths.
• Add erroring variants (`convertPaymentOptionE`, `convertTermInYearsE`, `convertLookbackPeriodE`) and wire them into the SP recommendation path in `parser_sp.go`.
• Leave the legacy non-erroring wrappers in place for `client.go` callers (RI recommendation path, owned by other PRs) with a deprecation comment pointing to the follow-up.
• Regression tests for every finding that FAIL pre-fix.