fix(google): map OpenAI tool results to Gemini user role

[BetterAndBetterII]

Description: (required)

• Google streamGenerateContent rejects OpenAI role: "tool" messages after they are rewritten to Google role function (HTTP 400: Role 'function' is not supported).
• Map tool results to user. The adapter already emits a native functionResponse part; Gemini accepts that part on a user turn, not on role function.
• Isolated to Google role mapping / tool-result handling. No other providers changed.

Fixes #1757

Tests Run/Test cases added: (required)

• transformOpenAIRoleToGoogleRole('tool') returns user and not the rejected function role
• Two-turn contents transform: assistant tool_calls → model functionCall; tool result → user functionResponse

Type of Change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)