Limit generated filename prefixes

[Sean-Kenneth-Doherty]

Summary

Fixes #409 by applying the generated filename prefix length cap inside _next_filename() before it checks for existing files. This keeps direct helper use from probing the filesystem with overlong title-derived names, while preserving the existing determine_path() cap.

Changes

• Add a shared generated-prefix limit constant.
• Clamp prefixes inside _next_filename() before Path.exists() and Path.glob() calls.
• Add regression coverage for direct _next_filename() calls with long prefixes and determine_path() with long figure titles.

Validation

• uv run --directory src/py pytest tests/test_path_tools.py -q -> 20 passed
• uv run --directory src/py ruff check kaleido/_utils/path_tools.py tests/test_path_tools.py -> passed
• uv run --directory src/py ruff format --check kaleido/_utils/path_tools.py tests/test_path_tools.py -> passed
• uv run --directory src/py python -m compileall -q kaleido/_utils/path_tools.py tests/test_path_tools.py -> passed
• git diff --check -> passed

[Sean-Kenneth-Doherty]

Fresh validation on the current branch head:\n\n- env TMPDIR=$PWD/.tmp-test uv run --directory src/py pytest tests/test_path_tools.py -q -> 20 passed\n- uv run --directory src/py ruff check kaleido/_utils/path_tools.py tests/test_path_tools.py -> passed\n- uv run --directory src/py ruff format --check kaleido/_utils/path_tools.py tests/test_path_tools.py -> passed\n- uv run --directory src/py python -m compileall -q kaleido/_utils/path_tools.py tests/test_path_tools.py -> passed\n- git diff --check origin/master...HEAD -> clean\n\nThe local worktree is clean after removing the temporary test directory.