fix(bedrockagentcore): relax allowlistedHeaders pattern to match CFN schema (#37969)

### Issue # (if applicable)

Closes #37964.

### Reason for this change

The `validateRequestHeaderConfiguration` method uses an outdated regex that only allows `Authorization` and `X-Amzn-Bedrock-AgentCore-Runtime-Custom-*` headers. The CloudFormation schema for `AWS::BedrockAgentCore::Runtime` has been updated and deployed to all regions to accept `^[A-Za-z][A-Za-z0-9_-]{0,255}$` (any valid HTTP header name).

This causes `cdk synth` to throw `InvalidRequestHeaderConfiguration` for valid headers that the service fully supports. Customers must use a CFN escape hatch to work around this.

### Description of changes

Updated the regex in `validateRequestHeaderConfiguration` from `/(Authorization|X-Amzn-Bedrock-AgentCore-Runtime-Custom-[a-zA-Z0-9-]+)/` to `/^[A-Za-z][A-Za-z0-9_-]{0,255}$/` in the stable module:

- **`aws-cdk-lib/aws-bedrockagentcore`** — `lib/runtime/runtime.ts`

The alpha module (`@aws-cdk/aws-bedrock-agentcore-alpha`) is deprecated and was not modified per maintainer guidance.

Policy enforcement (blocking restricted headers) is handled server-side in the control plane — the CDK construct only needs format validation.

### Describe any new or updated permissions being added

N/A — no IAM permission changes.

### Description of how you validated changes

- Added unit tests to the existing `Runtime request header configuration tests` block:
  - Headers beyond the old `X-Amzn-Bedrock-AgentCore-Runtime-Custom-` prefix (should pass)
  - Headers with invalid characters like spaces (should fail)
  - Headers starting with a number (should fail)
  - Headers with underscores (should pass)
- Added integration test with snapshot in the stable module location (`@aws-cdk-testing/framework-integ/test/aws-bedrockagentcore/test/agentcore/runtime/`)
- All 135 unit tests pass locally

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*