[Bug]: Gemini formatter splits parallel function responses into multiple user contents

## Describe the bug

Gemini function calling requires responses to parallel function calls to be returned together in one `Content` with `role: "user"`. `GeminiMessageConverter#convertMessages` currently appends an independent user content block for every `ToolResultBlock`.

## To Reproduce

1. Construct one `Msg` with `MsgRole.TOOL` and two `ToolResultBlock` instances.
2. Convert it with `GeminiMessageConverter#convertMessages`.
3. Observe two user contents, each containing one `FunctionResponse` part.

## Expected behavior

The converter should emit one `Content(role=user)` containing both `FunctionResponse` parts in their original order.

## Actual behavior

The converter emits one user content per tool result. This violates Gemini's parallel function-calling response grouping contract and can cause a replayed tool turn to be rejected or interpreted incorrectly.

## Scope

This is intentionally separate from #2335, which addresses Gemini thinking and thought-signature persistence. The issue concerns grouping of parallel tool results only.

## Proposed fix

Buffer function-response parts while converting one source `Msg`, then emit a single user content block before regular message parts. A regression test covers two tool results in one message.