[C#] fix: Remove [] intializer notation (#2232)

## Linked issues

closes: #minor

## Details
remove `[]` notation because it is not supported in version < C#12. That
includes .NET 6 which is a framework we target.

## Attestation Checklist

- [x] My code follows the style guidelines of this project

- I have checked for/fixed spelling, linting, and other errors
- I have commented my code for clarity
- I have made corresponding changes to the documentation (updating the
doc strings in the code is sufficient)
- My changes generate no new warnings
- I have added tests that validates my changes, and provides sufficient
test coverage. I have tested with:
  - Local testing
  - E2E testing in Teams
- New and existing unit tests pass locally with my changes

---------

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: Lily Du <[email protected]>
Co-authored-by: lilydu <[email protected]>
Co-authored-by: Tarek Mahmoud Sayed <[email protected]>
Co-authored-by: Steven Ickman <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Corina <[email protected]>
Co-authored-by: Yiqing Zhao <[email protected]>
Co-authored-by: Yiqing Zhao <[email protected]>
Co-authored-by: Alex Acebo <[email protected]>

dotnet/packages/Microsoft.TeamsAI/Microsoft.TeamsAI/.editorconfig

@@ -2,3 +2,6 @@
 
 # CS0618: Type or member is obsolete
 dotnet_diagnostic.CS0618.severity = silent
+
+# IDE0028: Simplify collection initialization
+dotnet_diagnostic.IDE0028.severity = silent

dotnet/packages/Microsoft.TeamsAI/Microsoft.TeamsAI/AI/Models/SequenceBuilder.cs

@@ -3,10 +3,10 @@
 
 namespace Microsoft.Teams.AI.AI.Models
 {
-    public class SequenceBuilder<T>
+    internal class SequenceBuilder<T>
     {
-        private Segment _first;
-        private Segment _last;
+        private Segment? _first;
+        private Segment? _last;
 
         public void Append(ReadOnlyMemory<T> data)
         {

dotnet/packages/Microsoft.TeamsAI/Microsoft.TeamsAI/AI/Models/StreamingChatToolCallsBuilder.cs

@@ -3,11 +3,11 @@
 
 namespace Microsoft.Teams.AI.AI.Models
 {
-    public class StreamingChatToolCallsBuilder
+    internal class StreamingChatToolCallsBuilder
     {
-        private readonly Dictionary<int, string> _indexToToolCallId = [];
-        private readonly Dictionary<int, string> _indexToFunctionName = [];
-        private readonly Dictionary<int, SequenceBuilder<byte>> _indexToFunctionArguments = [];
+        private readonly Dictionary<int, string> _indexToToolCallId = new();
+        private readonly Dictionary<int, string> _indexToFunctionName = new();
+        private readonly Dictionary<int, SequenceBuilder<byte>> _indexToFunctionArguments = new();
 
         public void Append(StreamingChatToolCallUpdate toolCallUpdate)
         {
@@ -39,7 +39,7 @@ public void Append(StreamingChatToolCallUpdate toolCallUpdate)
 
         public IReadOnlyList<ChatToolCall> Build()
         {
-            List<ChatToolCall> toolCalls = [];
+            List<ChatToolCall> toolCalls = new();
 
             foreach (KeyValuePair<int, string> indexToToolCallIdPair in _indexToToolCallId)
             {