Add addErrors/removeErrors/removeOperations/removeMembers transformations

[kubukoz]

Adds four transformations, following the existing addOperations / addMembers pattern.

addErrors / removeErrors

One trait each, selector :is(operation, service) — both shape types carry an errors property. On an operation the errors are specific to that operation; on a service they apply to every operation it contains.

Targets are constrained by @idRef(failWhenMissing: true, selector: "structure[trait|error]"), so a typo or a non-@error target fails at model-load time rather than during transformation.

addErrors skips errors already present on the shape, so it's idempotent.

removeOperations

Selector service, via the inherited EntityShape.Builder.removeOperation.

removeMembers

Selector :is(structure, union). Entries are plain member names rather than shape ids, matched case-insensitively — mirroring both Smithy's own member-name uniqueness rule and the existing addMembers behaviour. Shape builders have no removeMember, so the container is rebuilt from the surviving members.

Shared semantics for the removals

• Entries that aren't attached to the shape are ignored rather than an error, keeping the traits usable against upstream models that may or may not declare them.
• Only the containing shape's list is rewritten — the referenced operation/error shapes stay in the model, since something else may still bind them. Pair with smithy-build's own removeUnusedShapes to drop the orphans.

Two caveats, documented in the README and Javadoc

• Service-level errors can't be cancelled per-operation. Smithy has no opt-out for an error inherited from the enclosing service, so @removeErrors on an operation can't remove one the service contributes — it has to be removed from the service. Smithy semantics, not a limitation here.
• removeMembers only rewrites the container. Dropping a member something else depends on (an @httpLabel binding, a @required contract elsewhere) surfaces as a validation failure downstream rather than here.

Notes

Builder APIs were checked against the smithy-model jar rather than assumed — removeOperation and removeError live on different classes (EntityShape.Builder vs. the concrete builders), and AbstractShapeBuilder has no removeMember at all, which shaped the RemoveMembers implementation.

Test helpers are extracted into a shared TransformationSuite used by the four new suites. The three pre-existing suites each had their own copy and are left alone to keep the diff focused — worth collapsing separately.

70 tests pass, along with headerCheckAll, scalafmtCheckAll, mimaReportBinaryIssues and doc.

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