For data-plane, `azsdk pkg update-version` fails to update version/date when CHANGELOG.md contains (Unreleased)

After [autorest.typescript#3880](https://github.com/Azure/autorest.typescript/pull/3880) merges, the TypeSpec emitter will scaffold a `CHANGELOG.md` for new packages with the following format:
```
# Release History

## 1.0.0-beta.1 (Unreleased)

### Features Added

### Breaking Changes

### Bugs Fixed

### Other Changes
```

uses `azsdk_package_update_version` to update version and date in an existing `CHANGELOG.md` from autorest. the tool internally updates the version and release date in `CHANGELOG.md` by matching a **YYYY-MM-DD** date format in the version heading. When the file contains (**Unreleased**), the match fails and neither the version nor the date is updated — **no error is thrown and no warning is surfaced**.

Affected scenario: Management-plane packages that are newly scaffolded after PR #3880 merges, where the emitter creates a `CHANGELOG.md` with (**Unreleased**), and `azsdk pkg update-version` is used to stamp in a version and date before publishing.

Start with the implementation and tests for the `azsdk pkg update-version` command, then trace how it matches the version heading in `CHANGELOG.md`. Reproduce the issue using the `(Unreleased)` heading shown here and verify that the command updates both the package version and release date before publishing.

Written by the indexing model from the issue text.