feat: casing engine + schema_driven_xml transform (schema-driven provider machinery)

[jeffreyaven]

What

Provider-agnostic, opt-in machinery that lets specs move from per-op verbose Go templates to schema-driven, casing-aware handling. Everything is gated behind new per-spec flags/extensions, so existing providers (google, okta, github, awscc, ...) see zero behaviour change. Motivated by the native AWS provider; no AWS knowledge is hard-coded in core.

Two cleanly separable features (could be split into two PRs if preferred; bundled here):

1. casing engine (pkg/casing)

• Port of botocore's xform_name (snake) + inverses for pascal | kebab | camel, with a cache.
• Output: when config.snake_case_aliases: true, top-level response column names are snake-aliased at the SQL surface; the wire name is retained on the schema for response navigation.
• Input: when a method's request.nativeCasing is set (pascal | kebab | camel | snake), an unresolved snake SQL key is reverse-cased to the wire form and re-resolved (parameters and one level of request-body properties). Final miss yields a typed ParameterNotFoundError listing wire-format keys.
• Top-level only; nested struct contents are passed verbatim (matches the AWS CLI).

Wiring: const.go, config.go, provider.go (IsSnakeCaseAliasesEnabled), expectedRequest.go (GetNativeCasing), operation_store.go (GetRequestNativeCasing, reverse-casing in GetParameter/body matcher, GetParameterOrError), schema.go (getPropertiesColumns), radix_tree_address_space/legacy_address_space.go (GetColumns).

2. schema_driven_xml transform (pkg/stream_transform)

• New StreamTransformer family schema_driven_xml_v0.1.0 (distinct from golang_template_mxj_v*).
• Reads the response.schema_override schema, walks mxj-decoded XML, and projects rows into the {"<objectKey>": [...]} envelope. Decodes mxj WITHOUT casting and applies the schema's declared type, so 12-digit IDs stay strings, booleans stay booleans, objects/arrays are JSON-stringified, and self-closing elements become null.
• Skips the query / ec2 / rest-xml envelope based on info.x-protocol; handles list and singleton (incl. singleton-with-ancillary-list) shapes.
• The package stays free of internal/anysdk (consumes a neutral SchemaTree); the operation store adapts its Schema and dispatches in getOverridenResponse.

Tests

• pkg/casing: ToSnake (incl. acronyms), FromSnake for all 4 casings, round-trip, edge cases.
• pkg/stream_transform: ec2 list/small, query resultWrapper, query empty self-closing, rest-xml list/singleton/ancillary-list, type-dispatch, factory registration.
• internal: ParameterNotFoundError formatting.

Notes

• Additive + backward compatible: no flag / no extension => no behaviour change. Existing tests should pass unchanged.
• The mock-provider integration test and the robot tests live in the stackql repo (separate work item).
• GetOperationParameter (a secondary parameter path) was intentionally not extended; the primary GetParameter resolution carries reverse-casing.
• Built/verified by the engine packages' own unit tests; full go test ./... + golangci-lint to be run in CI.