Append handlers in-situ.

[plx]

While starting to read the code I noticed some code like this:

let oldValue = someDictionary[key, default:[]]
someDictionary[key] = oldValue + [valueToAppend]

I changed those sites to look like this:

someDictionary[key, default: []].append(valueToAppend)

Motivation and Context

The updated code may sometimes perform an in-situ append on the underlying storage; the original would always construct a new array.

In a worst-case scenario the original code could exhibit accidentally-quadratic behavior while, say, registering a large number of "handlers".

How Has This Been Tested?

I re-ran the unit tests and they still passed.

I also ran this test to double-check that the setter for subscript(key:default:) existed and worked as I thought:

@Test("In-Place Default Append Works")
func testInPlaceDefaultAppend() {
  var inSituAppend: [String: [Int]] = [:]
  var copyThenOverwrite: [String: [Int]] = [:]
  for value in 1...10 {
    for key in ["foo", "bar", "baz"] {
      inSituAppend[key, default: []].append(value)
      let oldValue = copyThenOverwrite[key, default: []]
      #expect(inSituAppend != copyThenOverwrite)
      copyThenOverwrite[key] = oldValue + [value]
      #expect(inSituAppend == copyThenOverwrite)
    }
  }
}

I didn't include that test in the PR because it's a one-off experiment to re-confirm my understanding of a standard-library method, but I'm happy to put it back in if you'd like.

Breaking Changes

No.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Glad there's an official sdk!