Lookup.ExplicitModifications switch to concrete dictionary type for enumerator.

[ttstanley]

Context

When building a smaller solution we can see the boxing to IEnumerator pop up for 0.1% allocations

Changes Made

Switched from returning IEnumerator to the underlying dictionary type to allow usage of the struct enumerator to prevent boxing.

[Copilot]

Pull Request Overview

This PR updates the ExplicitModifications property in Lookup.cs to return a concrete Dictionary instead of an IEnumerable, optimizing performance by leveraging a struct enumerator to prevent boxing.

• Changed the ExplicitModifications property return type from IEnumerable<KeyValuePair<string, MetadataModification>> to Dictionary<string, MetadataModification>.
• Aims to reduce allocations during enumeration by avoiding boxing.

Comments suppressed due to low confidence (1)

src/Build/BackEnd/Components/RequestBuilder/Lookup.cs:1183

• The XML documentation summary for ExplicitModifications still mentions 'an enumeration of the explicit metadata modifications'. Consider updating the summary to reflect that a Dictionary is returned, which may improve clarity for other developers.

            public Dictionary<string, MetadataModification> ExplicitModifications