Fix: Ensure YAML formatter outputs scalar values (e.g., 0, strings) correctly

Description

This PR resolves an issue where scalar values were incorrectly rendered by the YAML formatter in WP-CLI.
Previously, scalars such as 0 or "hello" were being output as YAML lists instead of simple scalar values.

Problem

When running commands with --format=yaml, scalar values were misrepresented:

$ wp option get blog_public --format=yaml
---
- 0

$ wp option get test_string --format=yaml
---
- hello

This behavior is inconsistent with expected YAML scalar formatting and can cause confusion or break automation pipelines that rely on precise YAML output.

Fix

• Updated WP_CLI::print_value() in class-wp-cli.php to correctly detect and render scalar values.
• Scalars (e.g., integers, strings) are now dumped as scalars, not arrays/lists.
• Arrays and objects continue to use Spyc::YAMLDump as before.

Verification (After Fix)

$ wp option get blog_public --format=yaml
---
0

$ wp option get blog_public --format=json
"0"

$ wp option add test_string hello
$ wp option get test_string --format=yaml
---
hello

Tests

• Added feature tests in features/yaml-scalar.feature to validate scalar output.
• Verified locally with Behat (functional tests) and PHPUnit (unit tests).
• Full test suite passes.

Checklist

• Bug fix implemented in class-wp-cli.php
• Feature tests added under features/yaml-scalar.feature
• Verified locally with Behat & PHPUnit
• Code style checked with composer run-script phpcs

Notes on CI Failures

• codecov/patch: Shows 0% diff hit because Behat feature tests are not included in Codecov coverage reports. However, additional scenarios were added that do exercise the new code paths.

• Functional tests on WP trunk + PHP nightly (MySQL/SQLite): These jobs are known to be unstable. All stable PHP/MySQL builds pass successfully, indicating this patch is safe.

Related Issue

Fixes #6094