feat: make msls_get_switcher() $attr optional

[apermo]

What

Gives the $attr parameter of msls_get_switcher() a default of array() so
the documented public template tag can be called with no arguments.

// includes/api.php
-function msls_get_switcher( $attr ): string {
+function msls_get_switcher( $attr = array() ): string {
     $arr = is_array( $attr ) ? $attr : array();
     $obj = apply_filters( 'msls_get_output', null );
     return ! is_null( $obj ) ? strval( $obj->set_tags( $arr ) ) : '';
 }

Closes #643.

Why

The body already coerces any non-array value to array(), so $attr was
effectively optional. But without a default, the natural template-tag call
msls_get_switcher() threw a fatal ArgumentCountError ("Too few arguments
… 0 passed and exactly 1 expected"). The sibling tag
msls_the_switcher( array $arr = array() ) already defaults, so the two were
inconsistent. A downstream theme calling msls_get_switcher() with no args
currently has to pass array() purely to satisfy the signature.

Backward compatibility

No behavior change for any existing caller:

• The parameter stays untyped so the is_array() coercion is preserved.
  This keeps the [sc_msls] shortcode
  (add_shortcode( 'sc_msls', 'msls_get_switcher' )) working — WordPress
  passes the empty string '' when a shortcode has no attributes, which an
  array type hint would have rejected.
• Existing msls_get_switcher( $attr ) calls are unaffected.
• The deprecated get_the_msls( $attr ) shim is unaffected.

Tests

Adds tests/phpunit/TestApi.php covering three cases:

• no-arg call → returns '' when no output object is registered;
• explicit attribute array → forwarded to set_tags();
• non-array value ('', the shortcode's no-attribute case) → coerced to
  array().

The test runs in a separate process so loading includes/api.php does not
collide with the Brain Monkey function stubs used elsewhere in the suite.
Full suite green (431 tests). composer phpcs passes; composer phpstan
shows no new findings (the pre-existing errors are in unrelated files).

The @since tag uses 3.0.0, the current unreleased version on master.

Disclosure

Claude (Claude Code) assisted with this change. I reviewed the diff, ran the
test suite, phpcs and phpstan locally, and take responsibility for the change.

[gemini-code-assist]

Code Review

This pull request updates the msls_get_switcher function in includes/api.php to make the $attr parameter optional, defaulting to an empty array, and updates its docblock documentation. Additionally, a new PHPUnit test class TestApi is introduced in tests/phpunit/TestApi.php to verify the behavior of msls_get_switcher under various scenarios, including calls without arguments, with an array, and with non-array values. There are no review comments, so we have no feedback to provide.

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