fix: Avoid modifying original config in AgentBuilder._build()

[Lingbo-Huang]

Related issue

Fixes #508

Problem

Currently, the AgentBuilder._build() method uses pop() operations which modify the original config dict, potentially causing:

1. Side effects: If the caller retains a reference to the config, they'll find it unexpectedly modified
2. Reusability issues: The same config object cannot be used multiple times
3. Unclear intent: The purpose of pop() is not clear enough

Use dictionary comprehension to create a new dict, filtering out fields that need to be passed explicitly:

# Filter out special fields that will be passed explicitly
# to avoid modifying the original config and parameter conflicts
config_for_init = {
    k: v for k, v in agent_config.items()
    if k not in ["sub_agents", "tools"]
}

agent = agent_cls(**config_for_init, sub_agents=sub_agents, tools=tools)

Changes:

Remove agent_config.pop("sub_agents")
Remove agent_config.pop("tools")
Add dictionary filtering to create config_for_init
Add comments explaining the change

Benefits

No side effects: doesn't modify original config
Reusable: config can be used multiple times
Clear intent: code purpose is explicit
Best practice: follows immutability principle

[Lingbo-Huang]

@cuericlee Hello, this is my first contribute in github. Thank you!