Components: document CSS module class composition

[ciampo]

What?

Follow up to #79442.

Clarifies the recommended clsx pattern for conditional CSS Module classes in packages/components/CONTRIBUTING.md.

Why?

Computed object keys can produce a literal undefined class when a CSS Module export is missing in tests.

How?

• Documents the safer condition && styles.className pattern.
• Updates the example to use that pattern with className as a direct clsx argument.
• Adds a packages/components changelog entry.

Testing Instructions

1. Run npm run lint:md:docs -- packages/components/CONTRIBUTING.md.

Testing Instructions for Keyboard

Not applicable; documentation-only change.

Screenshots or screencast

Not applicable.

Use of AI Tools

Created with assistance from ChatGPT/Codex and reviewed before opening this PR.

[ciampo]

Keeping the non-object form here, since the object form can produce undefined as an object key

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