Fix ModifiedTrait for traits with breaking change rules

[syall]

Overview

Fix ModifiedTrait for traits with breaking change rules.

Before this change, if a trait had breaking change rules defined, the diff strategy in ModifiedTrait would eventually resolve the trait as unknown and produce a ModifiedTrait warning on any change.

• Missing registering traits with breaking change rules:

  smithy/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ModifiedTrait.java

  Lines 148 to 151 in 7c6228f

  	} else if (definition.getBreakingChanges().isEmpty()) {
  	// Avoid duplicate validation events; only perform the default validation when there are no diff rules.
  	result.put(shape.getId(), DEFAULT_STRATEGIES);
  	}

• Results in treating traits with breaking change rules as unknown and warn on any change:

  smithy/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ModifiedTrait.java

  Lines 117 to 119 in 7c6228f

  	// If we don't know about the trait, warn on any change to it.
  	List<DiffStrategy> diffStrategies = strategies.computeIfAbsent(traitId,
  	t -> ListUtils.of(new DiffStrategy(DiffType.CONST, Severity.WARNING)));

Also, apply @tags(["diff.contents"]) to a structure member of the "all" ModifiedTrait test, which is a no-op.

Testing

Tested with @jsonName:

Model A:

$version: "2.0"

namespace ns.foo

structure Test {
    @jsonName("a")
    member: String
}

Model B:

$version: "2.0"

namespace ns.foo

structure Test {
    @jsonName("b")
    member: String
}

Diff events before the change (see duplicate ModifiedTrait event):

[ERROR] ns.foo#Test$member: Changed trait `smithy.api#jsonName` from `a` to `b` | TraitBreakingChange.Update.smithy.api#jsonName
[WARNING] ns.foo#Test$member: Changed trait `smithy.api#jsonName` from `a` to `b` | ModifiedTrait.Update.smithy.api#jsonName

Diff events after the change (no duplicate ModifiedTrait event):

[ERROR] ns.foo#Test$member: Changed trait `smithy.api#jsonName` from `a` to `b` | TraitBreakingChange.Update.smithy.api#jsonName

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[gosar]

can we add a test case?