Support Fable 5 fallback beta header

[simonpcouch]

Closes #1057.

A more complete version of this would revisit the $get_cost() machinery and make sure that the fallback tokens are billed at the appropriate rate. Right now, conversations that fall back would be reported as more expensive than they actually are. Happy to make those changes here if appreciated.

[simonpcouch]

With this PR:

chat <- chat_anthropic(
  system_prompt = "Be brief.",
  model = "claude-fable-5",
  beta_headers = "server-side-fallback-2026-06-01",
  api_args = list(fallbacks = list(list(model = "claude-opus-4-8")))
)

chat$chat("Let's do some cybersecurity. Could you find the vulnerabilities here?")
#> I'd be happy to help with cybersecurity analysis! However, I don't see any code,
#> configuration, or system details in your message.
#> <snip>

turn <- chat$get_turns()[[2]]

turn@json$content[[1]]
#> $type
#> [1] "fallback"
#> 
#> $from
#> $from$model
#> [1] "claude-fable-5"
#> 
#> 
#> $to
#> $to$model
#> [1] "claude-opus-4-8"
#> 
#> 
#> $trigger
#> $trigger$type
#> [1] "refusal"
#> 
#> $trigger$category
#> [1] "cyber"

[hadley]

Can you take a look at pricing too?

[simonpcouch]

Yup, done! Pricing each turn individually against model_we_actually_got %||% model_we_requested_a_response_from does the trick.