AI Gateway: Bedrock Converse API responses missing token usage and cost tracking

## Description

When routing AWS Bedrock requests through the AI Gateway, token usage and cost aren't tracked in the dashboard. Requests log correctly (status, model, latency), but usage shows "– in / – out" and cost shows "$ –".

## Cause

The Bedrock Converse API returns usage in camelCase, but the gateway only appears to parse snake_case fields:

**Bedrock response:**
```json
{
"usage": {
"inputTokens": 13,
"outputTokens": 12,
"totalTokens": 25
}
}
```

**OpenAI response (tracked correctly):**
```json
{
"usage": {
"input_tokens": 761,
"output_tokens": 3441,
"total_tokens": 4202
}
}
```

## Steps to Reproduce

1. Route Bedrock requests through AI Gateway using the documented URL rewriting approach
2. Make a request to any Bedrock model (e.g. Claude Sonnet 4.6)
3. Check the AI Gateway logs — tokens and cost are missing

## Related

- cloudflare/cloudflare-docs#20536 — Similar parsing issue with Anthropic direct API

Start at the AI Gateway's Bedrock Converse request and response handling, tracing how the documented URL-rewriting route maps response usage into dashboard logs. Compare the Bedrock camelCase usage object with the already-working OpenAI snake_case path, then reproduce with a Bedrock model. Done when input, output, total token usage, and cost appear instead of dashes.

Written by the indexing model from the issue text.