Fix fileBody and fileRangeBody in sttp4 stub server

[adamw]

Fixes #5440. Endpoints with .out(fileBody) interpreted by the sttp4 stub server crashed with IllegalArgumentException: Provided body: FileRange(...), cannot be adjusted to a file because the response encoder passed raw FileRange values to sttp's stub backend, which only accepts SttpFile objects.

The fix converts FileRange and InputStreamRange outputs to sttp-compatible types in the response encoder, and applies HTTP range selections during that conversion so stubbed responses carry the same bytes a real server would send. For files on the JVM, partial-range responses are materialized to a temporary file using file channels; on Scala.js, ranged file responses throw UnsupportedOperationException since that would require file-system access.

The SttpFileToTapirFile internal helper was renamed to SttpFileConversions and extended with reverse conversions (toSttpFile and fileAsInputStream), maintaining separate platform-specific implementations for JVM and JS. All existing tests pass, and new test coverage includes full-file responses, ranged file responses, and input stream ranges with and without bounds.

Closes #5440.

[adamw]

Great! I found it. The compilation for Scala 3 sttpStub4Server test sources is just warnings, not errors. The issue must be in the actual test run. Let me check what "fileBody" test is about in stp4 stub:

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CI Failure Summary for PR #5441

Three test jobs failed on JVM 11: Scala 2.12, 2.13, and 3. All jobs show the same error: sbt.TestsFailedException: Tests unsuccessful on the sttpStub4Server3 module.

Root Cause:
The sttpStub4Server tests (Scala 3 variant) failed after running 297 passing tests. The failure is in the test task of the sttpStub4Server3 / Test phase, which uses sttp4 (the stub4 backend for sttp client v4). Based on your recent changes (PR #5440 mentions "fileBody response in sttp4 stub interpreter"), the issue is likely in the fileBody handling for HTTP responses when using the stub interpreter.

Key Indicators:

• All 297 tests in Vertx/ZIO server tests pass successfully
• Failure occurs specifically when transitioning to sttpStub4Server3 tests
• The error doesn't print detailed test failure messages, only Tests unsuccessful

Most Likely Issue:
A recent change to handle fileBody responses in the sttp4 stub interpreter is causing compilation or test execution to fail. The error message indicates sbt detected test failure but the detailed output was either captured in thread dumps or lost during cleanup.