[Enhancement] : agents selection in AgentsDf would be better if it returns agent_idx/name as key

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when selecting agent from agentsDF the return comes as agent to value mapping
```python
w = model.agents["wealth"]
for k,v in w.items():
print(k)
```

Code to reproduce

```python
class ExampleAgentSet1(AgentSetPolars):
def __init__(self, model: ModelDF):
super().__init__(model)
self["wealth"] = pl.Series("wealth", [1, 2, 3, 4])
self["age"] = pl.Series("age", [10, 20, 30, 40])

def add_wealth(self, amount: int) -> None:
self["wealth"] += amount

def step(self) -> None:
self.add_wealth(1)

class ExampleAgentSet2(AgentSetPolars):
def __init__(self, model: ModelDF):
super().__init__(model)
self["wealth"] = pl.Series("wealth", [10, 20, 30, 40])
self["age"] = pl.Series("age", [11, 22, 33, 44])

def add_wealth(self, amount: int) -> None:
self["wealth"] += amount

def step(self) -> None:
self.add_wealth(2)

class ExampleAgentSet3(AgentSetPolars):
def __init__(self, model: ModelDF):
super().__init__(model)
self["age"] = pl.Series("age", [1, 2, 3, 4])
self["wealth"] = pl.Series("wealth", [1, 2, 3, 4])

def age_agents(self, amount: int) -> None:
self["age"] += amount

def step(self) -> None:
self.age_agents(1)

class ExampleModel(ModelDF):
def __init__(self, agents: AgentsDF):
super().__init__()
self.agents = agents

def step(self):
self.agents.do("step")

@pytest.fixture
def fix1_AgentSetPolars() -> ExampleAgentSet1:
return ExampleAgentSet1(ModelDF())

@pytest.fixture
def fix2_AgentSetPolars() -> ExampleAgentSet2:
return ExampleAgentSet2(ModelDF())

@pytest.fixture
def fix3_AgentSetPolars() -> ExampleAgentSet3:
return ExampleAgentSet3(ModelDF())

@pytest.fixture
def fix_AgentsDF(fix1_AgentSetPolars: ExampleAgentSet1,fix2_AgentSetPolars : ExampleAgentSet2, fix3_AgentSetPolars: ExampleAgentSet3) -> AgentsDF:
model = ModelDF()
agents = AgentsDF(model)
agents.add([fix1_AgentSetPolars,fix2_AgentSetPolars,fix3_AgentSetPolars])
return agents

@pytest.fixture
def fix1_model(fix_AgentsDF: AgentsDF) -> ExampleModel:
return ExampleModel(fix_AgentsDF)

model = fix1_model
w = model.agents["wealth"]
for k,v in w.items():
print(k)
```

please let me know if I am setting it up wrong

Start at the AgentsDF selection entry point used by model.agents["wealth"] and inspect how the supplied AgentSetPolars objects are mapped during selection. Run the reproduction to confirm the current keys, then define the expected agent_idx/name keys and add coverage for the returned mapping before verifying the example behavior.

Written by the indexing model from the issue text.