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`

coderd/x/chatd/chatprovider/chatprovider_test.go

@@ -1123,7 +1123,7 @@ func TestModelFromConfig_BedrockStreamingHeaders(t *testing.T) {
 	got := testutil.TryReceive(ctx, t, requests)
 	require.NoError(t, got.ReadError)
 	require.Equal(t, "/model/us.anthropic.claude-opus-4-6-v1/invoke-with-response-stream", got.Path)
-	require.Equal(t, "application/vnd.amazon.eventstream", got.Accept)
+	require.Empty(t, got.Accept)
 	require.Equal(t, "application/json", got.BedrockAccept)
 	require.Contains(t, got.Authorization, "AWS4-HMAC-SHA256")
 	require.Contains(t, got.Authorization, "x-amzn-bedrock-accept")