Enable mixing regular and built-in tools in Gemini 3+

[thisisnic]

Fixes #1054

The ability to combine built-in and existing tools was an addition in Gemini 3.

Previously, combining built-in and custom tools resulted in an error; this PR sets parameter includeServerSideToolInvocations to TRUE, which allows them to be combined.

Didn't add a test as it feels like more of a minor JSON configuration update than a true bug fix, but before/after are here.

Before:

library(ellmer)
chat <- chat_google_gemini()
#> Using model = "gemini-3.5-flash".
chat$register_tool(tool(
  function(x) x * 2,
  "Doubles a number",
  arguments = list(x = type_number("The number to double"))
))
chat$register_tool(google_tool_web_search())
chat$chat("What is double today's temperature in the UK?")
#> Error in `req_perform_connection()` at ellmer/R/httr2.R:55:5:
#> ! HTTP 400 Bad Request.
#> ℹ Please enable tool_config.include_server_side_tool_invocations to use
#>   Built-in tools with Function calling.

After:

library(ellmer)
chat <- chat_google_gemini()
#> Using model = "gemini-3.5-flash".
chat$register_tool(tool(
  function(x) x * 2,
  "Doubles a number",
  arguments = list(x = type_number("The number to double"))
))
chat$register_tool(google_tool_web_search())
chat$chat("What is double today's temperature in the UK?")
#> ◯ [tool call] tool_001(x = 21L)
#> ● #> 42
#> ◯ [tool call] tool_001(x = 33L)
#> ● #> 66
#> ◯ [tool call] tool_001(x = 34L)
#> ● #> 68
#> ◯ [tool call] tool_001(x = 23L)
#> ● #> 46
#> ◯ [tool call] tool_001(x = 70L)
#> ● #> 140
#> ◯ [tool call] tool_001(x = 94L)
#> ● #> 188
#> Currently in London, UK (as of ~7:00 AM BST), the temperature is approximately 
#> **21°C to 23°C** (around **70°F**), with a forecast high today reaching up to 
#> **33°C to 34°C** (around **91°F to 94°F**) [1, 2]. 
#> 
#> Depending on whether you are looking at the current temperature or today's 
#> forecast high (in Celsius or Fahrenheit), doubling the temperature yields the 
#> following:
#> 
#> ### 1. Doubling the Current Temperature (~21°C to 23°C / 70°F)
#> * **In Celsius:** Doubling 21°C to 23°C gives **42°C to 46°C**.
#> * **In Fahrenheit:** Doubling 70°F gives **140°F**.
#> 
#> ### 2. Doubling Today's Forecast High (~33°C to 34°C / 91°F to 94°F)
#> * **In Celsius:** Doubling 33°C to 34°C gives **66°C to 68°C**.
#> * **In Fahrenheit:** Doubling 91°F to 94°F gives **182°F to 188°F**.