Stripping function declarations on the last iteration when the history contains tool calls causes a ValidationError

### Description

**Summary**

FunctionInvokingChatClient removes every AIFunctionDeclaration from ChatOptions.Tools on the final iteration (added in [#7207](https://github.com/dotnet/extensions/issues/7207)) so the inner client can't return a tool call nobody will execute.

That shape is is invalid on Amazon Bedrock's Converse API, which requires toolConfig whenever any message contains a toolUse/toolResult block. This means that on Bedrock, every conversation that uses a tool and then reaches MaximumIterationsPerRequest fails with:

```
Amazon.BedrockRuntime.Model.ValidationException:
The toolConfig field must be defined when using toolUse and toolResult content blocks.
```

### Reproduction Steps

**Repro**

Any FunctionInvokingChatClient over AWS.Bedrock.MEAI with at least one AIFunction registered, driven until the iteration cap is hit.

1. iteration >= MaximumIterationsPerRequest → PrepareOptionsForLastIteration (FunctionInvokingChatClient.cs).
2. Every registered tool is an AIFunction, hence an AIFunctionDeclaration, so the tool is not AIFunctionDeclaration filter keeps nothing: remainingTools stays null, options.Tools = null, and options.ToolMode = null. https://github.com/dotnet/extensions/blob/5682e2bb5ab1c92ff0d1e2c828320ba97cff5d76/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs#L1078
3. The Bedrock client builds toolConfig only when `options?.Tools is { Count: > 0 }` (https://github.com/aws/aws-dotnet-ai/blob/main/src/AWS.Bedrock.MEAI/BedrockChatClient.cs#L931)), so ToolConfig stays null and the field is omitted from the payload.
4. Bedrock rejects the request: history has tool blocks, payload has no toolConfig.

### Expected behavior

There should be no ValidationException from Bedrock.

### Actual behavior

```
Amazon.BedrockRuntime.Model.ValidationException:
The toolConfig field must be defined when using toolUse and toolResult content blocks.
```

### Regression?

_No response_

### Known Workarounds

_No response_

### Configuration

_No response_

### Other information

_No response_

Start in src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs at PrepareOptionsForLastIteration and the filtering logic around line 1078. Reproduce a conversation that reaches MaximumIterationsPerRequest with an AWS.Bedrock.MEAI client and inspect the outgoing request after toolUse/toolResult history exists. Done means the capped final request is accepted by Bedrock without the reported ValidationException.

Written by the indexing model from the issue text.