fix: detect tool calls in mixed content responses for GoogleGenAiChatModel

[VedantMadane]

Summary

Fixes #5466

GoogleGenAiChatModel.responseCandidateToGeneration used allMatch to detect function call responses, requiring all parts to be function calls. When Gemini returns a mix of text/thinking parts and functionCall parts (e.g. with thinking enabled), the check evaluated to false and tool calls were silently dropped.

Changes:

• Changed allMatch to anyMatch so function calls are detected even when mixed with text or thought parts
• Text content from non-function-call parts is now preserved in the AssistantMessage alongside the tool calls (previously, mixed responses resulted in empty content)
• Added regression tests covering: mixed text + tool calls, multiple function calls with text, pure text, and pure function call responses

Before (broken):

// ALL parts must be function calls — fails on mixed responses
candidate.content().get().parts().get().stream()
    .allMatch(part -> part.functionCall().isPresent());

After (fixed):

// ANY part being a function call triggers tool call extraction
candidate.content().get().parts().get().stream()
    .anyMatch(part -> part.functionCall().isPresent());

[lazer-dev]

this is duplication of #5468
also your fix is wrong, concatenation of content is not correct way to fix this issue