fix(llm): keep tool calls in Responses API assistant messages that have content - #5941

Summary

ResponsesAPIMessageSerializer (used by ChatAzureOpenAI for Responses API models) converts an assistant message's tool calls to text because EasyInputMessageParam has no tool call field. It only did this when content was None. If the assistant message had both content and tool calls, the tool calls were silently dropped from the model input.

Changes

• Tool calls are now appended to the assistant content whenever they exist. For string content the text is added after a newline. For content parts an extra input_text part is appended.
• Messages with no tool calls serialize as before.

Tests

Added two cases to tests/ci/models/test_azure_responses_api.py: string content with tool calls, and text-part content with tool calls.

pytest tests/ci/models/test_azure_responses_api.py: 2 failed, 15 passed, 2 skipped before the change; 17 passed, 2 skipped after. ruff check and ruff format --check are clean.

Summary by cubic

Fixes ResponsesAPIMessageSerializer silently dropping tool calls when an assistant message has both content and tool calls. Tool calls are now appended to the assistant content instead of only being serialized when content is None.

• Tool calls are appended after a newline for string content, or as an extra input_text part for content parts.
• Messages with no tool calls serialize as before.

^{Written for commit e5fc798. Summary will update on new commits.}