Fixes #851 : relevanceThreshold is not utilized preventing additional…

… memory items (#920)

fix bug related to issue #851.
Unused parameter relevanceThreshold is passed to SearchAsync method to
ensure only relevant documents are fetched.

### Motivation and Context

Please help reviewers and future users, providing the following
information:
1. The code change is related because current code retrieves all memory
items regardless of similarity which prevents adding new memory items.
Check SemanticChatMemoryExtractor.ExtractCognitiveMemoryAsync.
2. The issue is only the first memory item is preserved. Additional
memory items are lost because they always seem to be similar to the
first memory item.
3. With this fix, a memory item is skipped only if there is similar
memory item in the memory. As a result distinct memory items will be
preserved.
  4. It fixes an open issue #851 .

### Description

The chance is simple, it utilizes unutilized relevanceThreshold
parameter in ISemanticMemoryClientExtensions.SearchAsync method.
SemanticChatMemoryExtractor.ExtractCognitiveMemoryAsync sends an upper
threshold to find similar items, but since it is not used all non
relevant memory items are returned.

### Contribution Checklist

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [?] All unit tests pass, and I have added new tests where possible. I
could not find any unit tests to run or extend. Please point out if
there are any tests relevant to memory retrieval.
- [X] I didn't break anyone 😄

webapi/Extensions/ISemanticMemoryClientExtensions.cs

@@ -99,7 +99,7 @@ await memoryClient.SearchAsync(
                 indexName,
                 filter,
                 null,
-                0,
+                relevanceThreshold, // minRelevance param
                 resultCount,
                 cancellationToken);