Tests: Missing direct unit test coverage for isHandle() in wporg-handle.cjs

Description

src/wporg-handle.cjs exports isHandle(value) to determine whether a value is already a canonical WordPress.org handle before using it in structural positions (such as patch filenames in src/patch-provenance.cjs).

Currently, tests/unit/wporg-handle.test.cjs does not import or test isHandle(value) directly. While parseHandle and handoffFilename have test coverage, isHandle itself lacks dedicated unit tests verifying its exact contract:

• Valid canonical handles (letters, digits, valid internal separators _, -, .).
• Strict lowercase requirement (JaneDoe must return false).
• Length boundaries (MAX_HANDLE_LENGTH = 60 returns true, 61 returns false).
• Refusal of leading/trailing separators (-janedoe, janedoe-, .janedoe, janedoe_).
• Refusal of invalid characters (spaces, slashes, path traversal, newlines, #).
• Refusal of non-string or empty inputs (null, undefined, numbers, objects, arrays, empty strings).

Proposed Solution

Add dedicated unit tests in tests/unit/wporg-handle.test.cjs to cover all criteria of isHandle().

Start in src/wporg-handle.cjs to inspect the exported isHandle(value), then open tests/unit/wporg-handle.test.cjs and follow the existing parseHandle and handoffFilename test style. Import isHandle directly, cover the listed valid and invalid inputs plus length boundaries, and run the unit test file to confirm the complete contract is exercised.