…ilent defaults (#1085)

* fix(aws/savingsplans): error on unknown payment-option/term, no silent default

convertPaymentOption and convertTermToSeconds previously logged a warning
and silently substituted All Upfront (largest cash outlay) or 1 year when
given an unrecognized or empty value, risking a mis-purchase on any typo or
new/renamed AWS enum value.

Both functions now return an explicit error on unrecognized input so the
purchase and offering-lookup paths fail loud instead of buying the wrong
product. Mirrors the pattern already used by the RDS convertPaymentOption.

Regression tests for H1 (unknown/empty payment option) and H2 (unknown/empty
term) are added to TestClient_FindOfferingID_AllPaymentOptions and
TestClient_FindOfferingID_TermVariations; the previously-passing "default term"
case is updated to assert the error path.

Fixes H1 and H2 from docs/code-review/19-hardcoded-fallbacks-aws.md (PR #1039).

* fix(aws): fail loud on unknown payment-option/term/engine

Fixes H3, H4, M1-M6, L1, L2 from the 19-hardcoded-fallbacks-aws.md
audit plus H1/H2 from the now-merged #1039 branch (savingsplans).

H3 (converters.go): add convertPaymentOptionE/convertTermInYearsE/
convertLookbackPeriodE that error on unrecognized input; wire into the
SP recommendation path (parser_sp.go). The legacy non-erroring wrappers
are retained for the RI path in client.go (owned by #865/#1075) with a
deprecation comment and a follow-up reference.

H4 (elasticache/client.go): convertPaymentOption now returns (string, error)
mirroring the RDS sibling that already errors correctly.

H1/H2 (savingsplans/client.go): convertPaymentOption and
convertTermToSeconds now return (type, error); cherry-picked from the
merged #1039 branch where the fix was staged.

M4 (parser_services.go, rds/client.go): parseRDSDetails no longer
silently defaults AZConfig to "single-az" when CE omits DeploymentOption;
findOfferingID rejects empty AZConfig with an explicit error.

M5 (parser_services.go): resolveEC2Tenancy now maps only the known CE
values ("shared"/nil -> default, "dedicated" -> dedicated); any other
value (e.g. "host" for Dedicated Hosts) returns an error rather than
silently collapsing to default tenancy.

M6 (rds/client.go): normalizeEngineName now returns (string, error) and
errors for ambiguous Oracle ("oracle*"), SQL Server ("sqlserver*"/"sql-server*"),
and bare Aurora inputs; unambiguous engines (mysql, postgresql, mariadb,
aurora-mysql, aurora-postgresql) pass through unchanged.

M1/M2/M3 (ec2/client.go): replace raw "convertible" literals with
types.OfferingClassTypeConvertible; buildEC2OfferingQuery errors on empty
Platform (purchase path); centralize "Linux/UNIX" literal into
defaultEC2Platform constant (exchange-path helpers retain their safe
fallback; the purchase path does not).

Regression tests added for H3, H1/H2, H4, M2, M4, M5, M6, L1, L2; each
fails pre-fix and passes post-fix.

Closes #1084

* test(aws/rds): clarify ambiguous-engine regression test

Simplify TestNormalizeEngineName_AmbiguousErrors: remove the confusing
oracle-se2 case (strings.Contains("oracle-se2","oracle") is true, so it
errors like all other oracle inputs -- no special case needed) and remove
the misleading inline comment. All remaining cases assert that any Oracle,
SQL Server, or bare Aurora input errors, which is the invariant under test.

* refactor(aws): extract helpers to fix gocyclo violations in rds and savingsplans

gocyclo -over 10 fails on two functions added by the fail-loud refactor:
- savingsplans.(*Client).findOfferingID (complexity 12): extract
  resolveSPPlanType and buildSPOfferingsInput helpers
- rds.(*Client).paginateRDSOfferings (complexity 11): extract
  fetchRDSOfferingPage to isolate the per-page API call and scan

Also apply gofmt to rds/client.go (trailing whitespace / brace style).
Behaviour and error paths are unchanged; all rds and savingsplans
tests still pass.

* fix(aws/rds): tighten AZ-config and engine-name validation (CR #1085)

Three related fail-loud fixes in the AWS RDS purchase path:

1. parseRDSDetails: unknown DeploymentOption now errors instead of
   silently folding into "single-az". Only "Multi-AZ" and "Single-AZ"
   are accepted; any other non-nil value (e.g. "Multi-AZ-Readable-Standbys")
   returns an explicit error so the bad token surfaces rather than
   driving findOfferingID to the wrong RI class.

2. findOfferingID: AZConfig validation now uses a switch with an
   explicit default error case rather than only checking for empty string.
   A non-empty but invalid value (typo, future AWS enum addition) would
   previously fall through to multiAZ==false in paginateRDSOfferings,
   silently treating the bad input as single-AZ.

3. normalizeEngineName: Oracle and SQL Server checks changed from
   strings.Contains to == so that edition-qualified tokens (oracle-se2,
   oracle-ee, sqlserver-se, sqlserver-web, etc.) pass through as valid
   RDS ProductDescription values. The previous Contains check contradicted
   the error message itself, which told callers to supply those exact
   edition tokens.

Regression tests added for all three: each new test was verified to
fail against the pre-fix code and pass after.