KNOX-3443: add unit test coverage for MimeTypes

[Raghav-Mah3shwari]

What changes were proposed in this pull request?

This patch adds comprehensive unit test coverage for
org.apache.knox.gateway.util.MimeTypes.

The tests cover:

• MIME type creation and parsing
• Null MIME type handling
• Charset insertion, replacement, and preservation
• Existing MIME type parameters
• Malformed MIME type input
• Explicit and default charset lookup
• Case-insensitive and whitespace-tolerant MIME type matching
• Unknown, empty, and parameterized MIME types
• All configured default MIME type mappings

How was this patch tested?

Executed the following command from the repository root:

mvn -pl gateway-util-common -Dtest=MimeTypesTest test

Integration Tests

No integration tests were added or modified. This change only adds unit tests for existing utility behavior and does not introduce a new feature, external integration, or workflow change.

UI changes

Not applicable. This patch does not include UI changes.

[github-actions]

Test Results

  4 files    4 suites   40s ⏱️
 84 tests  84 ✅ 0 💤 0 ❌
102 runs  102 ✅ 0 💤 0 ❌

Results for commit 29f8e7b.