Support TOML as alternative to YAML for test cases

[hidai25]

What

Allow writing test cases in .toml files in addition to .yaml/.yml.

Why

Some teams prefer TOML (used by pyproject.toml, Rust ecosystem). Supporting it is low-effort since Python 3.11+ has built-in tomllib.

Scope

• Extend TestCaseLoader to handle .toml files
• Add .toml to the directory glob pattern
• Add one example test case in TOML format to docs/examples

Getting Started

• Loader implementation: evalview/core/loader.py lines 17-64
• Currently YAML-only: line 32 (yaml.safe_load(f))
• File glob patterns: line 8-14 (CONFIG_FILE_PATTERNS)
• Use tomllib (Python 3.11+) or tomli as fallback for 3.9/3.10

Acceptance Criteria

• .toml test files are loaded alongside .yaml files
• Existing YAML tests still work (no breaking change)
• One example .toml test case added
• Works on Python 3.9+ (use tomli fallback if tomllib unavailable)