fix(bedrock_agentcore_starter_toolkit): type mismatch: api_key_env_var_name declared optional[str] but defaults to false (bool)

## Description

The field `api_key_env_var_name` is annotated as `Optional[str]` but its default value is `False`, which is a `bool`. This is a silent type violation. Any downstream code that performs string operations on this field (e.g., `os.environ[api_key_env_var_name]` or string formatting) will fail at runtime when the default is used. With mypy configured in strict mode (`disallow_untyped_defs`, `warn_return_any`), this will also be flagged as a type error. The intended default is almost certainly `None` (meaning "not set").

**Severity**: `medium`
**File**: `src/bedrock_agentcore_starter_toolkit/create/types.py`

## Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.

Open src/bedrock_agentcore_starter_toolkit/create/types.py and inspect the api_key_env_var_name declaration, including its annotation and default. Confirm the default is consistent with Optional[str], then run the project's type checks or tests to verify the mismatch is resolved.

Written by the indexing model from the issue text.