fix(gemini): trailing empty text part with thoughtSignature

[HhSen]

What type of PR is this?

fix: there is trailing empty text candidate part with thoughtSignature instead of placing the signature in the first part of functionCall (as described in the document https://docs.cloud.google.com/vertex-ai/generative-ai/docs/thought-signatures)

Check the PR title.

• fix(gemini): trailing empty text part with thoughtSignature

(Optional) Translate the PR title into Chinese.

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
fix: there is trailing empty text candidate part with thoughtSignature instead of placing the signature in the first part of functionCall (as described in the document https://docs.cloud.google.com/vertex-ai/generative-ai/docs/thought-signatures) like:

"candidates": [
            {
                "content": {
                    "parts": [
                        {
                            "functionCall": {
                                ...
                            }
                        },
                        {
                            "text": "",
                            "thoughtSignature": "...thoughtSignature here..."
                        }
                    ],
                    "role": "model"
                },
                "finishReason": "STOP",
                "groundingMetadata": {
                    "retrievalMetadata": {},
                    "searchEntryPoint": {
                        "renderedContent": "..."
                    },
                    "webSearchQueries": [
                        ...
                    ]
                }
            }
        ],

added fallback handle to move the thoughtSignature in the empty part to the first tool_call part

zh(optional):

(Optional) Which issue(s) this PR fixes:

fixed: #756

(optional) The PR that updates user documentation:

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