Issue #594: fix demo chart scan when the project path contains a space

[timmolter]

Fixes #594.

The bug

The reporter's classpath was E:\FEE\2nd Term\XChart\... — note the space in 2nd Term. That space is the whole story:

1. DemoChartsUtil resolved the chart package directory with new File(url.getFile()). URL.getFile() returns a percent-encoded path, so the space came back as %20 and the File pointed at a directory that does not exist.
2. getClassesByFile returned an empty list on !dir.exists() — silently. No charts discovered.
3. XChartDemo.createNodes therefore built a tree with no children, leaving the root node "XChart Example Charts" (a String user object) as a leaf.
4. Clicking it hit valueChanged, which casts any leaf's user object to ChartInfo → ClassCastException: String cannot be cast to ChartInfo.

Verified directly:

url                              = file:/tmp/2nd%20Term/foo/
new File(url.getFile()).exists() = false
new File(url.toURI()).exists()   = true

This also explains the issue title — the tree looks empty, so users see no "file explorer".

Changes

• Resolve through URI, not getFile() — for both the file and jar branches. The jar path is itself a URL, so it goes through new File(new URL(...).toURI()) rather than being chopped at the first /, which keeps spaces and Windows drive letters intact. The JarFile is now closed via try-with-resources (dropping the @SuppressWarnings("resource")).
• Fail loudly — getAllDemoCharts() used to return null when the package was missing (an NPE waiting to happen in createNodes) and an empty list when nothing was found. Both now throw IllegalStateException with the offending URL, so this is diagnosable instead of mysterious.
• Guard the cast — valueChanged now checks nodeInfo instanceof ChartInfo. Category and root nodes carry a String, and either can be a leaf; that should not kill the EDT.
• Null-guard on dir.listFiles().

Test

DemoChartsUtilTest copies the compiled demo classes into a temp directory whose name contains a space, points a parent-less URLClassLoader at it, and asserts the scan still finds charts. Confirmed red before the fix and green after:

# before
java.lang.IllegalStateException: No demo charts were found at:
  file:/var/folders/.../xchart%20demo%20classes.../org/knowm/xchart/demo/charts
# after
Tests run: 81, Failures: 0, Errors: 0, Skipped: 0

Two things worth flagging

• DemoChartsTest had stopped compiling — it calls new ToolTips(chart), but that constructor now takes (Chart, boolean, ToolTipType). Repaired here as a one-liner, since it blocked running any test in this module.
• The reason nobody noticed is that xchart-demo sets <maven.test.skip>true</maven.test.skip> in its pom and CI never overrides it, so none of this module's tests — including the new one — run in CI. I left that alone as out of scope for this issue, but it's worth deciding on separately: the comment says it exists to skip test compilation during release:prepare, which a release-profile-scoped setting could handle without disabling the tests everywhere.

Verified locally with mvn -pl xchart-demo test -Dmaven.test.skip=false: 81 tests, all passing.

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