Add missing periods to tables and lists in MSTest analyzer documentation

[Copilot]

Review feedback identified missing periods in table cells and list items across MSTest analyzer rule category documentation, violating the Microsoft writing style guide requirement for proper punctuation of complete sentences.

Changes made:

• Added periods to all table cell entries containing complete sentences (rule titles) across 4 category files
• Added periods to all bullet list items with complete sentences
• Files updated: design-rules.md, performance-rules.md, suppression-rules.md, usage-rules.md

Example:

Before:

| [MSTEST0004](mstest0004.md) | Public types should be test classes | Info | Yes |

- **[MSTEST0004](mstest0004.md)**: Keep helper classes internal, only test classes should be public

After:

| [MSTEST0004](mstest0004.md) | Public types should be test classes. | Info | Yes |

- **[MSTEST0004](mstest0004.md)**: Keep helper classes internal, only test classes should be public.

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Internal previews

📄 File	🔗 Preview link
docs/core/testing/mstest-analyzers/design-rules.md	docs/core/testing/mstest-analyzers/design-rules
docs/core/testing/mstest-analyzers/performance-rules.md	docs/core/testing/mstest-analyzers/performance-rules
docs/core/testing/mstest-analyzers/suppression-rules.md	docs/core/testing/mstest-analyzers/suppression-rules
docs/core/testing/mstest-analyzers/usage-rules.md	docs/core/testing/mstest-analyzers/usage-rules