Change warning when setting model.agents to error

[EwoutH]

Replaced the warning with an AttributeError when attempting to set model.agents. This change enforces that the agents attribute will be reserved for internal use by Mesa in future releases. Users must update their code to use a different attribute name for custom agent storage.

This allows us to start using model.agents in Mesa 3.0 for ourselves, which helps with #2212.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 -2.1% [-2.4%, -1.9%]	🔵 +0.8% [+0.6%, +1.0%]
Schelling	large	🔵 -2.5% [-3.1%, -1.9%]	🔵 -0.6% [-3.2%, +1.6%]
WolfSheep	small	🔵 -1.2% [-1.3%, -1.0%]	🔵 -1.0% [-1.1%, -0.8%]
WolfSheep	large	🔵 -2.1% [-2.5%, -1.8%]	🔵 +0.4% [+0.2%, +0.6%]
BoidFlockers	small	🔵 +1.0% [+0.2%, +1.7%]	🔵 +0.2% [-0.5%, +1.0%]
BoidFlockers	large	🔵 +0.7% [+0.3%, +1.2%]	🔵 +0.1% [-0.7%, +0.9%]

[EwoutH]

@projectmesa/maintainers I would like this in the next pre-release. Would somebody be so kind to review?