fix: replace deprecated seed= with rng= in all example models

[ospreyboi]

Fixes part of #273
All three example models (negotiation, sugarscrap_g1mt, epstein_civil_violence) were using the deprecated seed= parameter in both model.py and app.py files. This causes it to create a FutureWarning in Mesa 3.5 and will break in Mesa 4.0.
This replaces all occurrences of seed=None with rng=None in the function signatures and super().init(seed=seed) with super().init(rng=rng) in all three examples.
All 285 tests pass after the changes made.

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[wang-boyu]

Thanks for sending the PR. I'd prefer to have it as part of #273 when we migrate to Mesa 4. Currently mesa-llm has mesa pinned <4 (#272).

Another reason is that switching examples to rng= causes a Solara seed-reset issue on Mesa 3.x. It is fixed in Mesa 4 (see mesa/mesa#3518) but has not been backported to any 3.x release yet. If a future Mesa 3.x release backports this fix, we can revisit merging this earlier before #273, since it's also compatible with Mesa 3.