test: consolidate cli validation coverage

[sunt05]

Summary

• consolidate test/core/test_cli_validation.py from 12 overlapping tests down to 5 focused tests
• drop two fragile cases that were testing platform-specific file behaviour rather than SUEWS logic
• keep the existing helper coverage and the UTF-8 report unit test

What changed

• merged the repeated minimal-invalid-YAML assertions into one test that checks output creation, non-empty files, and meaningful report content
• merged the re-run scenarios into a single three-run test covering both same-input and updated-YAML paths without truncation
• merged the issue #1097 fixture checks into one test that preserves the non-empty and meaningful-content assertions
• merged the Windows-path coverage into one cross-platform test with platform-specific YAML setup
• removed the unused os and stat imports

Testing

• python3 -m compileall test/core/test_cli_validation.py
• pytest test/core/test_cli_validation.py -v
  Blocked locally because test/conftest.py imports supy, and the current environment fails loading f90wrap with missing symbol _sizeof_fortran_t_
• uv run --extra dev pytest test/core/test_cli_validation.py -v
  Blocked locally by dependency resolution for sphinx-design>=0.7.0 against the project requires-python >=3.9

Closes #1237

[github-actions]

CI Build Plan

Changed Files

Tests (1 file)

• test/core/test_cli_validation.py

Build Configuration

	Configuration
Platforms	Linux x86_64
Python	3.9, 3.14
Test tier	smoke (critical tests only)
UMEP build	Skipped (no ABI changes)
PR status	Draft (reduced matrix)

Rationale

• Test files changed -> validation build
• No compiled extension changes -> UMEP build skipped (nightly provides coverage)

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