Add configurable center obstacle size to Pursuit

[paranoa233]

Description

Adds a center_box_size=(width, height) argument to the Pursuit environment, measured in grid cells. None preserves the existing proportional center obstacle, while (0, 0) creates an obstacle-free map.

The change also:

• validates the configured dimensions and rejects maps with no free cells;
• adds coverage for rectangular, zero-sized, default, and invalid configurations;
• bumps Pursuit from v5 to v6 and updates the registry, manual policy, parameter combinations, and documentation.

Fixes #1382

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (all hooks pass for the changed files; the all-files mixed-line-ending hook rewrites unrelated files on Windows)
• I have run pytest -v and no errors are present (targeted Pursuit and registry tests pass: 58 tests; the broader optional-environment files require Atari native dependencies unavailable in this Windows environment)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings generated by the targeted tests that are related to my code
• I have added tests that prove my feature works
• New and existing Pursuit unit tests pass locally with my changes

Validation

• All pre-commit hooks pass for the ten changed files.
• 58 targeted Pursuit and environment-registry tests pass.
• PettingZoo AEC API test passes with a custom center box.
• PettingZoo Parallel API test passes with a custom center box.

[jkterry1]

Hey thank you, this needs some notes in the documentation updating it for this

[jkterry1]

@paranoa233 I apologize for the hassle, but could you please rebase your changes off main?

[virgilt]

@paranoa233 there's merge conflicts.

[paranoa233]

Rebased the PR onto the latest main and resolved the merge conflicts. The focused Pursuit test suite passes locally (12 tests). The new GitHub Actions runs are currently awaiting maintainer approval.