get_class_name incorrectly matches .net/.org/.io sub-packages inside com.* packages

## Description

`get_class_name()` in `java/private/package.bzl` produces an incorrect `test_class` when a `com.*` Java package contains a sub-package named `net`, `org`, `io`, `ai`, `co`, `me`, or `dev`.

This is related to #104 but is a **different failure mode**: #104 is about uncommon TLDs not being recognized at all. This bug is about a known TLD prefix (`.net.`, `.org.`, etc.) being matched **inside** a `com.*` package name instead of at the top level.

## Example

Source path: `com/example/net/NetTest.java`

1. `get_class_name` converts it to `com.example.net.NetTest`
2. Scans `_PREFIXES = (".com.", ".org.", ".net.", ...)`
3. `.com.` is checked first via `name.find(".com.")` — but the string starts with `com.` (no leading dot), so it returns `-1`
4. `.net.` matches at position 12 inside `com.example.net.NetTest`
5. Returns `net.NetTest` instead of `com.example.net.NetTest`

Result: `Class not found: [net.NetTest]`

## Minimal reproduction

Unzip the attached `rules-jvm-bug-poc.zip` and run:

```bash
bazel test //src/test/java/...
```

**Expected:** Both tests pass.
**Actual:** `NetTest` fails with `Class not found: [net.NetTest]`, `UtilsTest` passes.

Reproduction project structure

```
├── .bazelrc # Java 17 toolchain
├── .bazelversion # 9.0.0
├── BUILD.bazel
├── MODULE.bazel # contrib_rules_jvm 0.32.0
└── src/test/java/
├── BUILD.bazel # java_test_suite with two tests
└── com/example/
├── net/NetTest.java # FAILS
└── utils/UtilsTest.java # PASSES
```

`src/test/java/BUILD.bazel`:
```python
load("@contrib_rules_jvm//java:defs.bzl", "java_test_suite")

java_test_suite(
name = "tests",
srcs = [
"com/example/net/NetTest.java",
"com/example/utils/UtilsTest.java",
],
deps = ["@maven//:junit_junit"],
)
```

## Root cause

In [`java/private/package.bzl`](https://github.com/bazel-contrib/rules_jvm/blob/main/java/private/package.bzl#L20-L31):

```python
_PREFIXES = (".com.", ".org.", ".net.", ".io.", ".ai.", ".co.", ".me.", ".dev.")

def get_class_name(package, src, prefixes=[]):
idx = src.rindex(".")
name = src[:idx].replace("/", ".")
# ...
for prefix in prefixes:
idx = name.find(prefix)
if idx != -1:
return name[idx + 1:]
```

`name.find(".com.")` fails when the string **starts** with `com.` because there's no dot at position -1. The scan then falls through to `.net.` which matches inside the string.

## Workaround

Pass `package_prefixes` to anchor the match:

```python
java_test_suite(
...
package_prefixes = [".com.example."],
)
```

## Affected packages

Any `com.*` (or `org.*`, etc.) package that has a sub-package matching one of the entries in `_PREFIXES`: `net`, `org`, `io`, `ai`, `co`, `me`, `dev`.

Start in java/private/package.bzl at get_class_name and review how package prefixes are matched. Unzip the reproduction project and run bazel test //src/test/java/... to confirm the NetTest failure. Done means both NetTest and UtilsTest pass without the incorrect net.NetTest class name.

Written by the indexing model from the issue text.