fix: match Bedrock streaming accept headers (#24781)

> Mux is working on behalf of Mike.

## Summary
- Bump `github.com/coder/anthropic-sdk-go` to the corrected Bedrock
streaming header fix from coder/anthropic-sdk-go#14.
- Match botocore's `InvokeModelWithResponseStream` request shape by
using `X-Amzn-Bedrock-Accept` and omitting the HTTP `Accept` header.
- Update chatd regression coverage for the corrected header shape.

## Context
The previous fix set `Accept: application/vnd.amazon.eventstream`. Real
boto3/botocore streaming requests do not send that header. They send
`X-Amzn-Bedrock-Accept: application/json`, which is the modeled Bedrock
request header for the desired model response MIME type.

## Validation
- `go test ./coderd/x/chatd/chatprovider -run
'TestModelFromConfig_Bedrock(StreamingHeaders|StripsAnthropicHeaders)?$'
-count=1`
- `go mod tidy -diff`
- `git diff --check`
- pre-commit hook during `git commit`

go.mod

@@ -91,8 +91,8 @@ replace charm.land/fantasy => github.com/coder/fantasy v0.0.0-20260426185602-951
 
 // coder/coder uses a fork of charmbracelet's fork of the Anthropic Go SDK
 // with performance improvements and Bedrock header cleanup.
-// See: https://github.com/coder/anthropic-sdk-go/commits/67514346e0fb
-replace github.com/charmbracelet/anthropic-sdk-go => github.com/coder/anthropic-sdk-go v0.0.0-20260428111347-67514346e0fb
+// See: https://github.com/coder/anthropic-sdk-go/commits/47cab198e449
+replace github.com/charmbracelet/anthropic-sdk-go => github.com/coder/anthropic-sdk-go v0.0.0-20260428122333-47cab198e449
 
 // Replace sdks with our own optimized forks until relevant upstream PRs are merged.
 // https://github.com/anthropics/anthropic-sdk-go/pull/262