CI: Add metadata.toml validation workflow (implements #3272)

[Vanya-kapoor]

Summary

Adds automated CI validation for metadata.toml files in all examples,
as discussed in #3272.

Changes

• .github/workflows/validate_metadata.yml: GitHub Actions workflow that
  runs on every PR touching metadata files
• scripts/validate_metadata.py: Validation script that checks:
  • All required fields are present
  • space is one of: Grid, Network, Continuous, None
  • time is one of: Discrete, Continuous
  • complexity is one of: Basic, Advanced
  • Valid TOML syntax

Why

As raised in #3272 without validation, metadata can silently drift
from the agreed standard as new examples are added. This ensures
every PR with a metadata.toml is automatically checked.

Related

• Implements schema validation proposed by @areyoush and @ShreyasN707 in #3272 Investigating approaches for reviving mesa-examples #313
• Works alongside the metadata.toml additions in my PR

[codebreaker32]

See my comment in #423

[B2prakash]

Nice work on the CI validation! A few thoughts:

1. The validation script looks solid — checking required fields, valid TOML syntax, and enum values for space/time/complexity covers the key cases.

2. Question about the space values — should "None" be a valid option? Some examples like pure network models might not have a spatial component, but I'm curious if maintainers want that as an explicit value or just make the field optional.

3. The workflow trigger — running only on PRs touching metadata files is smart, keeps CI fast. Does it also run on the scheduled checks, or is that planned separately?

4. One concern — the PR has multiple unrelated commits mixed in (forest_fire fixes, AgentSet.to_list fix, DataCollector import fix). These should probably be in separate PRs to keep review focused. The metadata validation is its own feature.