ci: add test workflow token permissions

[kitsuyui]

Summary

• Add explicit read-only contents permission to the test workflow.
• Keep the workflow behavior unchanged while avoiding repository-default token permissions.

Verification

• git diff --check
• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.12 -color .github/workflows/test.yml
• collateral check: clean
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo check
• cargo test --all-targets --all-features
• cargo llvm-cov --lcov --output-path coverage.lcov
• cargo tree --duplicates (0 duplicates)

[github-actions]

Code Metrics Report

	main (0c91c43)	#117 (f468216)	+/-
Coverage	68.3%	68.3%	0.0%
Code to Test Ratio	1:0.1	1:0.1	0.0
Test Execution Time	3s	6s	+3s

Details

  |                     | main (0c91c43) | #117 (f468216) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          68.3% |          68.3% | 0.0% |
  |   Files             |              4 |              4 |    0 |
  |   Lines             |            550 |            550 |    0 |
  |   Covered           |            376 |            376 |    0 |
  | Code to Test Ratio  |          1:0.1 |          1:0.1 |  0.0 |
  |   Code              |            593 |            593 |    0 |
  |   Test              |             71 |             71 |    0 |
- | Test Execution Time |             3s |             6s |  +3s |

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Reported by octocov