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: https://github.com/smithy-lang/smithy/blob/7c6228f351d85dab343af614181bbaa41893737e/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ModifiedTrait.java#L148-L151
• Results in treating traits with breaking change rules as unknown and warn on any change: https://github.com/smithy-lang/smithy/blob/7c6228f351d85dab343af614181bbaa41893737e/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ModifiedTrait.java#L117-L119

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

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