feat(amazon-bedrock): return request body from doGenerate and doStream

[dontgitit]

Summary

BedrockChatLanguageModel.doGenerate() and doStream() do not return a request field, so consumers that rely on the request body (observability frameworks, onFinish callbacks, etc.) receive undefined. There was an existing // TODO request? comment at the doStream return site.

This PR adds request: { body: args } to both methods, matching the pattern used by @ai-sdk/anthropic:

 // doGenerate return
 return {
   content,
   finishReason: { ... },
   usage: convertBedrockUsage(response.usage),
+  request: { body: args },
   response: { ... },
   warnings,
 };

 // doStream return
 return {
   stream: response.pipeThrough(...),
-  // TODO request?
+  request: { body: args },
   response: { headers: responseHeaders },
 };

args is the Bedrock Converse API command object (the same value passed as body to postJsonToApi), so it captures the full request payload including model config, messages, tools, and inference parameters.

Motivation

Observability frameworks like Mastra use the request field from provider doStream()/doGenerate() to populate tracing span inputs (e.g., model_step.input). With the Bedrock provider, this is always undefined, resulting in empty span attributes — making it impossible to see what was sent to the LLM.

The @ai-sdk/anthropic provider already returns request: { body: args } from both methods (ref).

Fixes #13927
Related: #5129