Make test fixtures portable to non-Maven build layouts

[hectorcast-db]

What

Make TestOSUtils.resource() copy a test fixture — a single file or a whole
directory subtree, preserving its path — into a writable temp directory and
return that path, instead of chmod-ing the resource in place. Route
DatabricksConfigTest.testConfigFileScopes through the same helper instead of
hardcoding a relative HOME.

Why

TestOSUtils.resource() did getResource(file) → new File(path).setExecutable(true).
That only works when test resources are exploded on disk as writable files
(Maven's target/test-classes). When the SDK is built and tested outside Maven —
for example with Bazel, where resources are served from a jar or a read-only tree —
File.setExecutable returns false and the tests fail.
DatabricksConfigTest.testConfigFileScopes had the same fragility: it hardcoded
HOME to the relative path src/test/resources/testdata, which only resolves when
the working directory is the module root.

The new helper handles both file: and jar: resource URLs and preserves the
resource's path (some tests assert on substrings like
testdata/corrupt/.databrickscfg). Behavior under mvn test is unchanged.

Tests

mvn test in CI. Verified out-of-band that DefaultProfileTest,
DatabricksConfigTest, and DatabricksAuthManualTest — which previously could not
locate/execute their fixtures outside Maven's layout — pass when the suite is built
with Bazel. The change is behavior-preserving for the Maven build.

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-java

Inputs:

• PR number: 897
• Commit SHA: 4d8e8e61474519319c921b9a2c55ed8dddcb0fdc

Checks will be approved automatically on success.