resolve OpenRouter tool calling failures

[simonpcouch]

Closes #889.

  library(ellmer)

# for an example tool that has some non-trivial arguments
library(btw)

client <- chat_openrouter(model = "anthropic/claude-haiku-4.5")
client$set_system_prompt("Be very terse.")
client$register_tool(btw_tool_files_list_files)
client$register_tool(btw_tool_files_code_search)

client$chat("Is there a file in the R directory with util in the name?")
#> ◯ [tool call] btw_tool_files_list_files(path = "R", type = "file", regexp =
#> "util", ...)
#> ● #> | path | type | size | modification_time |
#>   #> |------|------|------|-------------------|
#>   #> | R/chat-utils.R | file | 336 | 2025-11-26 10:14:30 |
#>   #> | R/utils-S7.R | file | 4.71K | 2025-11-26 10:14:30 |
#>   #> | R/utils-auth.R | file | 2.13K | 2025-11-26 10:14:30 |
#>   #> …
#> Yes, there are several files with "util" in the name in the R directory:
#> 
#> - `chat-utils.R`
#> - `utils-S7.R`
#> - `utils-auth.R`
#> - `utils-callbacks.R`
#> - `utils-cat.R`
#> - `utils-coro.R`
#> - `utils-merge.R`
#> - `utils-prettytime.R`
#> - `utils.R`
client$chat("How about util in the contents of any .R files?")
#> ◯ [tool call] btw_tool_files_code_search(term = "util", limit = 100L,
#> case_sensitive = FALSE, ...)
#> ℹ Indexing files in '/Users/simoncouch/Documents/rrr/helperbench/inst/resources…
#> ✔ Indexing files in '/Users/simoncouch/Documents/rrr/helperbench/inst/resources…
#> 
#> ● #> [{"filename":"piny-loon_reprex.spin.R","size":482,"last_modified":"2025-1…
#> Yes, "util" appears in many .R files. Key occurrences include:
#> 
#> - **R/utils-*.R files** (multiple files with "Utility functions" comments)
#> - **utils:: calls** throughout (e.g., `utils::packageVersion()`, 
#> `utils::capture.output()`, `utils::modifyList()`)
#> - **Comments** like "Utilities" and "Utility functions"
#> 
#> The search returned 33 matches across various R files, mostly referencing 
#> utility functions from R's `utils` package or comments describing utility 
#> functionality.

^{Created on 2025-11-26 with reprex v2.1.1}

[hadley]

Hmmmm, actually would you mind adding a to_json() helper and then using it every where. It looks like this might also affect a couple of other providers.