sam init: add Java durable-function starter templates

The [development environment guide](https://github.com/aws/aws-durable-execution-docs/pull/231) documents that `sam init` includes durable-function starter templates for TypeScript, Python, and Java.

On PR #231, @bchampp noted that [aws-sam-cli-app-templates](https://github.com/aws/aws-sam-cli-app-templates) does not currently include Java durable templates:

> I don't see any templates in https://github.com/aws/aws-sam-cli-app-templates for Java. @zhongkechen could you look into adding some templates here please?

**Action:** Add Java durable-function starter templates to `aws-sam-cli-app-templates` so the guide's `sam init` flow holds for Java.

**Doc dependency:** Until Java templates land, the "Write Function" section of the development environment page may overstate Java `sam init` availability and should be softened if this is not resolved before that page publishes.

Ref: PR #231 review comment https://github.com/aws/aws-durable-execution-docs/pull/231#discussion_r3692749481

Start by inspecting the existing durable-function templates for TypeScript and Python in aws-sam-cli-app-templates, then compare them with the Java conventions used by that repository. Add the corresponding Java starter templates and verify that the sam init flow exposes them; ensure the development environment guide's Write Function section accurately reflects the result.

Written by the indexing model from the issue text.