support thinking tokens from `chat_claude()`

[simonpcouch]

On main, I see:

library(ellmer)

chat_claude(
  model = "claude-3-7-sonnet-latest",
  api_args = list(thinking = list(type = "enabled", budget_tokens = 2000))
)$chat("hey!")
#> ! Unknown delta type "thinking_delta".
#> ! Unknown delta type "thinking_delta".
#> ! Unknown delta type "thinking_delta".
#> ! Unknown delta type "signature_delta".
#> Hi there! How can I help you today? Feel free to ask me any questions or let me
#> know what you'd like to chat
#> Error in `FUN()`:
#> ! Unknown content type "thinking".
#> ℹ This is an internal error that was detected in the ellmer package.
#>   Please report it at <https://github.com/tidyverse/ellmer/issues> with a
#>   reprex (<https://tidyverse.org/help/>) and the full backtrace.

^{Created on 2025-04-01 with reprex v2.1.1}
The above seems to enable thinking just fine, though we just don't yet know how to handle that content type. Mentioned in #394.

[hadley]

Duplicate of #340