Update bedrock example for AI Gateway

[captainvera]

Summary

This PR changes the AI Gateway bedrock code example.

closes #14492

Requests to AWS bedrock require the request to be signed. The previous example was doing so incorrectly by signing the request without its body. This PR fixes that so the example works out of the box.

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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PR Change Summary

Updated the AI Gateway bedrock code example to correctly sign requests with their body.

• Corrected the signing process for AWS bedrock requests to include the request body.
• Updated the example to use a dynamic URL based on the region.
• Modified the request handling to fetch through the AI Gateway URL.

Modified Files

• src/content/docs/ai-gateway/providers/bedrock.mdx

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