.Net: Fix non-streaming function calling text and usage aggregation

[Cozmopolit]

Motivation and Context

Fixes #13420

When using auto function invocation in non-streaming mode (GetChatMessageContentAsync), intermediate text content generated by the LLM before tool calls is silently discarded. Additionally, token usage is not aggregated across multiple API calls in the auto-invoke loop.

Problem: If the LLM responds with "Let me check that for you..." before requesting a tool call, and then provides a final answer after the tool result, only the final answer is returned. The intermediate text is lost.

Scenario: Users relying on non-streaming mode with auto function invocation expect to receive all text the LLM generated, not just the final response.

Description

This PR modifies the non-streaming auto function calling loop in all three affected connectors to:

1. Aggregate text content across all loop iterations using a StringBuilder
2. Aggregate token usage (InputTokens + OutputTokens) across all API calls
3. Apply aggregated state to the final response, including when a filter terminates early

Affected Connectors:

Connector	File
OpenAI / Azure OpenAI	Connectors.OpenAI/Core/ClientCore.ChatCompletion.cs
Google / Gemini	Connectors.Google/Core/Gemini/Clients/GeminiChatCompletionClient.cs
MistralAI	Connectors.MistralAI/Client/MistralClient.cs

Implementation approach:

• Before the loop: Initialize aggregatedContent (StringBuilder) and token counters
• Each iteration: Accumulate text (with \n\n separator) and token counts
• On exit: Prepend aggregated text to final message and add AggregatedUsage metadata (only when multiple iterations occurred)

Out of Scope:

• MEAI-based connectors (Azure AI Inference, Ollama) - they already handle this correctly via FunctionInvokingChatClient
• Python SDK - separate issue/PR needed

New Tests

Added 5 unit tests for the OpenAI connector in FunctionCallingContentAggregationTests.cs:

Test	Description
NonStreaming_IntermediateTextBeforeToolCall_IsAggregatedInFinalResponseAsync	Verifies text before tool calls is preserved
NonStreaming_TokenUsage_IsAggregatedAcrossAllIterationsAsync	Verifies AggregatedUsage metadata contains sum of all tokens
NonStreaming_SingleIteration_NoAggregationMetadataAddedAsync	Verifies no regression for single-iteration calls
NonStreaming_ToolCallWithoutIntermediateText_OnlyFinalTextReturnedAsync	Verifies empty intermediate text is handled correctly
NonStreaming_FilterTerminatesEarly_AggregatedContentStillAppliedAsync	Verifies aggregation works when filter terminates

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄