KNOX-3317: Fixed NPE when no token-metadata-headers are declared for ICEBERG-REST

[smolnar82]

KNOX-3317 - Add unit tests for TokenMetadataHeaderHandler

What changes were proposed in this pull request?

This PR introduces comprehensive unit tests for the TokenMetadataHeaderHandler class within the gateway-service-restcatalog module.
Specifically, it adds tests to:

• Verify that no NullPointerException occurs when the token-metadata-headers configuration parameter is missing (null).
• Ensure headers are correctly applied to outbound requests based on inbound client credentials and token metadata.
• Validate support for custom header prefixes.
• Confirm graceful handling of missing metadata items.

How was this patch tested?

The patch was tested by running the newly created unit tests and existing tests in the gateway-service-restcatalog module:
mvn -pl gateway-service-restcatalog test

The test suite TokenMetadataHeaderHandlerTest includes:

• testNullTokenMetadataHeadersConfig: Verifies the fix for the NPE.
• testEmptyTokenMetadataHeadersConfig: Verifies handling of empty configuration strings.
• testApplyHeaders: Verifies correct mapping of multiple metadata items to headers.
• testApplyHeadersCustomPrefix: Verifies that custom header prefixes are respected.
• testApplyHeadersMissingMetadata: Verifies that headers are not added if metadata is missing.

Integration Tests

N/A

UI changes

(None)

[github-actions]

Test Results

21 tests   21 ✅  1s ⏱️
 1 suites   0 💤
 1 files     0 ❌

Results for commit 49bd998.