feat(gemini): support mixing provider tools with custom function tools

[cainydev]

Closes #972

Gemini 3+ models support mixing built-in provider tools (e.g. google_search, url_context) with custom function declarations in the same request via toolConfig.includeServerSideToolInvocations. Previously Prism threw an exception in all three handlers when this was attempted.

Changes

• Remove the exception thrown when provider tools and custom tools are used together
• Automatically set includeServerSideToolInvocations: true in tool_config when both tool types are present
• Apply the same fix to the searchGrounding provider option in the Stream handler, which is the same scenario
• Extract a ToolConfigMap to centralise tool_config building across Text, Structured, and Stream handlers

[jamsouf]

+1