Refactor JBangOperation to use ProcessExecutor, add timeout/IO controls, and harden validation

- Switch to `ProcessExecutor` instead of raw `ProcessBuilder`
    - Add timeout support with configurable limit
    - Add configurable I/O handling for inherited vs captured output
- Harden input validation for all setters and `execute()`
- Validate `jbang` executable exists and is executable before running
- Update `fromProject()` to preserve explicitly set values
- Improve error handling
    - Add distinct failure on timeout
    - Check `result.isSuccess()` before returning
    - Honor `exitOnFailure` flag consistently
- Clarify `reset()` behavior to preserve `workDir`, `jBangHome`, and `timeout`

Author: ethauvin

example/src/bld/java/com/example/ExampleBuild.java

@@ -19,7 +19,7 @@ public ExampleBuild() {
 
         downloadSources = true;
 
-        repositories = List.of(MAVEN_CENTRAL, RIFE2_RELEASES, RIFE2_SNAPSHOTS);
+        repositories = List.of(MAVEN_CENTRAL, RIFE2_RELEASES);
 
         var junit = version(6, 0, 3);
         scope(test)