feat: add env_str to snippet_client_v2.Config to allow am instrument command prefix (fixes #1004)

[devimano2011]

Fixes #1004

Problem

Users who need to prepend environment variables or command wrappers (e.g. ENV_VAR=value or a wrapper command) to the am instrument command in snippet_client_v2 had no way to do so.

Solution

Add an env_str field to snippet_client_v2.Config that, when set, is prepended to the am instrument command. This allows users to:

• Set environment variables: config.env_str = 'MY_VAR=value'
• • Use command wrappers: config.env_str = 'wrapper_cmd'

Changes

• Added env_str: str = '' field to the Config dataclass with documentation
• • Updated _LAUNCH_CMD template to support {env_str} prefix
• • Pass env_str value when building the launch command in _start_server()

Usage

config = snippet_client_v2.Config(env_str='MY_VAR=hello')
ad.load_snippet('maps', 'com.example.maps', config=config)