test(showcase): add integration tests for resumable upload

[whowes]

Work in progress, not ready for review

[gemini-code-assist]

Code Review

This pull request introduces a new integration test class, ITResumableUpload, to verify the behavior of the generated ResumableUploadServiceClient across various scenarios, including synchronous/asynchronous uploads, multi-chunk uploads, and channel guards. The review feedback suggests a minor improvement to use Files.newInputStream(file) instead of new FileInputStream(file.toFile()) to ensure compatibility with non-default file systems.

[gemini-code-assist]

Prefer using Files.newInputStream(file) instead of new FileInputStream(file.toFile()). Calling Path.toFile() can throw an UnsupportedOperationException if the Path is associated with a non-default file system (such as an in-memory file system like Jimfs). Using Files.newInputStream(path) is the idiomatic Java NIO.2 approach and ensures compatibility across different FileSystem implementations. This improvement can be applied to all other occurrences in this file (lines 106, 127, 141, 156, and 185).

Suggested change

	try (InputStream stream = new FileInputStream(file.toFile())) {
	try (InputStream stream = Files.newInputStream(file)) {