fix: prevent IllegalStateException when ToolResponseMessage has no responses (GH-6197)

[suryateja-g13]

Problem

With the new OpenAI Java SDK, ChatCompletionToolMessageParam.builder().build() now throws IllegalStateException: toolCallId is required at build time if toolCallId is not set. The previous code had two bugs that trigger this:

1. Empty responses path: when ToolResponseMessage.getResponses() is empty, the old code called builder.build() without ever setting toolCallId.
2. Builder reuse: a single builder instance was shared across all responses in the stream, meaning state from a previous response (e.g. a null ID) could bleed into the next.

The ToolCallAdvisor introduced in M7 auto-registers when tools are configured, changing the tool call flow and exposing both of these latent bugs.

Fix

• Stream through toolMessage.getResponses(), skipping entries where id is null/empty.
• Create a fresh ChatCompletionToolMessageParam.Builder per response to eliminate builder-reuse state leakage.
• Remove the separate empty-responses early-return branch (now handled naturally by the filtered stream returning an empty list).

Tests

Three new unit tests added to OpenAiChatModelTests:

• toolResponseMessageWithPopulatedResponses_mapsToOneParamPerResponse — verifies each response maps to one tool message param.
• toolResponseMessageWithEmptyResponses_producesNoMessages — verifies an empty ToolResponseMessage produces zero tool-typed params.
• toolResponseMessageWithNullId_skipsResponse — verifies responses with null IDs are filtered out.

Fixes #6197