…#25221)

Adds middleware in `withAWSBedrockOptions` that appends the AWS Partner
Revenue Measurement (PRM) attribution string to the User-Agent header on
every Bedrock API call made through AI Bridge.

This is the AI Bridge counterpart to the Terraform provisioner change
merged in #23138. Together, they ensure all AWS API calls made by Coder
(both workspace infrastructure via Terraform and LLM inference via
Bedrock) include PRM attribution.

## How it works

- A middleware is added before `bedrock.WithConfig(awsCfg)` that reads
the existing `User-Agent` header and appends
`sdk-ua-app-id/APN_1.1%2Fpc_cdfmjwn8i6u8l9fwz8h82e4w3%24`
- Only affects Bedrock calls; OpenAI and direct Anthropic API calls are
unaffected
- Uses `option.WithMiddleware` rather than `option.WithHeader` because
the existing User-Agent (set by the Anthropic SDK) must be preserved and
appended to, not replaced

## Tests

- **Positive**: `TestAWSBedrockIntegration` verifies PRM attribution is
present in the User-Agent on Bedrock requests
- **Negative**: `TestAnthropicMessages` verifies PRM attribution is
absent on non-Bedrock requests

## References

- Companion Terraform provisioner PR: #23138 (merged)
- Backport: #24052 (merged)
- Preserve existing `AWS_SDK_UA_APP_ID`: #24606 (open)
- Original `coder/aibridge` PR:
coder/aibridge#224 (superseded by this PR since
aibridge was moved into coder/coder via #24190)
- [AWS SDK Application ID
docs](https://docs.aws.amazon.com/sdkref/latest/guide/feature-appid.html)
- [AWS PRM Automated User
Agent](https://prm.partner.aws.dev/automated-user-agent.html) (partner
login required)

> Generated with [Coder Agents](https://coder.com/agents)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
(cherry picked from commit 094fe97)