`ChatBedrock()` mantle APIs: default model isn't served, and cross-region prefix isn't stripped from requests

When porting #345 to ellmer in https://github.com/tidyverse/ellmer/pull/1101/, I found an edge case; 🤖 summary below:

Two related problems with model ids on the messages and responses APIs.

ChatBedrock(api="messages") with no model uses DEFAULT_MODEL (us.anthropic.claude-sonnet-4-6), but mantle's Anthropic endpoint doesn't serve Sonnet 4.6, with or without the prefix. list_models() on that endpoint currently returns only claude-fable-5, claude-haiku-4-5, claude-opus-4-7, claude-opus-4-8, claude-opus-5, and claude-sonnet-5.

import chatlas as ctl

ctl.ChatBedrock(api="messages").chat("Say hi")
#> 404: The model 'us.anthropic.claude-sonnet-4-6' does not exist

Separately, CROSS_REGION_PREFIX is only stripped for the MODEL_APIS lookup, not from the model id in the request. Converse needs the prefix but mantle rejects it, so a prefixed id that routes to mantle 404s:

ctl.ChatBedrock(model="us.openai.gpt-5.4").chat("Say hi")
#> 404: The model 'us.openai.gpt-5.4' does not exist

ellmer handles both by defaulting per API and stripping the prefix from the request when the API is messages or responses (aws_strip_region_prefix() in R/provider-aws.R).

Tested against chatlas main (0.22.1.dev5).

Start at ChatBedrock's model defaulting and request construction for the messages, responses, and Converse APIs, then compare the behavior with list_models(). Use the two reproductions in the issue to verify that the appropriate mantle models are selected and that prefixed model IDs work for both API paths.