Update noop-api module to follow style guide

[Copilot]

This PR makes minimal changes to align the noop-api module with the style guide requirements defined in docs/style-guide.md.

Changes Made

1. Added final keyword to public classes

Per the style guide requirement that "Public non-internal classes should be declared final where possible":

• NoopOpenTelemetry → public final class NoopOpenTelemetry
• NoopContextStorageProvider → public final class NoopContextStorageProvider

2. Optimized static imports in test

Following the style guide recommendations for static imports:

• Added import static java.util.concurrent.TimeUnit.DAYS;
• Removed unused import java.util.concurrent.TimeUnit;
• Used DAYS instead of TimeUnit.DAYS in the test method

Verification

• All existing tests continue to pass
• Build and spotless formatting checks pass
• Changes are minimal (only 4 lines modified across 3 files)
• No breaking changes to functionality or public API

The noop-api module now fully complies with the project's style guide requirements.

Fixes #272.

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