[BUG] Unable to set default request header allowlist value in CLI in non-interactive move for Configure

**Describe the bug**
When trying to automate the configure command in non-interactive move, it is not possible to set the default values or run the configure command without the`--request-header-allowlist` flag.

It is not possible to remove this flag as it gets prompted otherwise. We should either have an option to allow default values, or remove the interactive prompt altogether as the function [prompt_request_header_allowlist](https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/65e7ff208c91f5d01c0fa9d8d12deebc016adda5/src/bedrock_agentcore_starter_toolkit/cli/runtime/configuration_manager.py#L206) defaults to None anyway.

**To Reproduce**
Steps to reproduce the behavior:
1. Run configure in in non-interactive mode by providing all required values as flags
```
agentcore configure \
--entrypoint "agent.py" \
--name "agent" \
--requirements-file "requirements.txt" \
--execution-role "$AGENT_RUNTIME_ROLE_ARN" \
--ecr "$ECR_REPOSITORY_URI" \
--authorizer-config null \
--disable-memory
```
2. Request header allowlist is prompted

**Expected behavior**
The configuration process should begin having the default request header allowlist values set.

**Environment:**
- OS: macOS 15.6.1
- Python version: 3.12.12
- Package version: 0.1.32
- Installation method: pip

**Additional context**
Add any other context about the problem here.

Start in src/bedrock_agentcore_starter_toolkit/cli/runtime/configuration_manager.py at prompt_request_header_allowlist. Run the non-interactive agentcore configure command from the issue and trace why the prompt still appears when the flag is omitted. Done means configuration proceeds with the default request header allowlist values without prompting.

Written by the indexing model from the issue text.