[FEATURE] Claude apps gateway: Support for Bedrock Guardrails headers

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

I work with multiple enterprise customers who deploy Claude apps gateway and would like to use Bedrock Guardrails but putting ANTHROPIC_CUSTOM_HEADERS in a managed.policies[].cli.env block just pushes that env var down to the Claude Code client, where it has the exact same problem as setting it locally: those headers are added to the request the client sends to the gateway, and the gateway strips them when it re-originates its own call to Bedrock.

Looking at the [gateway protocol reference](https://code.claude.com/docs/en/llm-gateway-protocol#request-headers): ANTHROPIC_CUSTOM_HEADERS values do arrive at the gateway as headers on the client's request to the gateway. But the [Bedrock upstream section](https://code.claude.com/docs/en/claude-apps-gateway-config#amazon-bedrock) shows the gateway builds its own signed InvokeModel call to Bedrock from region, auth, and base_url there's no documented field that copies arbitrary inbound client headers onto that outbound Bedrock call. The only fields the docs describe crossing that Anthropic→Bedrock translation boundary are the anthropic_beta/anthropic_version body fields (per the [format-translation table](https://code.claude.com/docs/en/llm-gateway-protocol#api-formats)), not headers generally, and definitely not a guardrail header.

### Proposed Solution

Forwarding the ANTHROPIC_CUSTOM_HEADERS sent by the client when the gateway builds its own signed InvokeModel call to Bedrock

### Alternative Solutions

_No response_

### Priority

Critical - Blocking my work

### Feature Category

Configuration and settings

### Use Case Example

_No response_

### Additional Context

_No response_

Start with the gateway protocol reference’s request-headers section and the Amazon Bedrock upstream configuration, then trace how ANTHROPIC_CUSTOM_HEADERS are handled when the gateway creates its signed InvokeModel call. Done means the Bedrock request preserves the client’s guardrail headers without changing the documented format translation behavior.

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