Relax Custom Header regex to allow new pattern

[tejaskash]

Description

https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-header-allowlist.html

Steps to Reproduce

See link

Expected Behavior

https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-header-allowlist.html

Actual Behavior

https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-header-allowlist.html

CLI Version

No response

Operating System

macOS

Additional Context

No response