🧪 [testing improvement] Add tests for TusServletRequest trailer header fallback

[tomdesair]

🎯 What: The TusServletRequest was missing test coverage for the getHeader(String name) fallback logic. Specifically, when super.getHeader(name) returns a blank value or null, the class is supposed to look up the value in trailerHeaders.
📊 Coverage: Added test cases in TusServletRequestTest to cover standard header retrieval, fallback to trailerHeaders when the header is missing, fallback to trailerHeaders when the header is blank, and when a header does not exist in standard or trailer headers.
✨ Result: The test coverage for TusServletRequest.java has been improved, and the previously untested method is fully covered.

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PR created automatically by Jules for task 8727877931933897901 started by @tomdesair

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[coveralls]

Coverage is 94.595% — feature/add-tus-servlet-request-test-8727877931933897901 into master. No base build found for master.