Signature: Check for positive case in host support

[obenland]

Proposed changes:

• Renamed and inverted logic of rfc9421_is_unsupported to could_support_rfc9421 in Signature class for clarity.
• Updated all usages and related tests to reflect the new method name and logic.

Other information:

• Have you written new tests for your changes, if applicable?

[Copilot]

Pull Request Overview

Updates the Signature class to use a positively named method for RFC-9421 support checks and adjusts tests accordingly.

• Renamed and inverted the logic of rfc9421_is_unsupported to could_support_rfc9421
• Updated call sites in sign_request and the private support method
• Adjusted tests to reflect the new method name and inverted assertions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/includes/class-test-signature.php	Swapped reflections and assertions to use could_support_rfc9421
includes/class-signature.php	Renamed method and inverted its return logic for clarity

Comments suppressed due to low confidence (2)

tests/includes/class-test-signature.php:640

• [nitpick] The test method name still references rfc9421_is_unsupported. Rename it to test_could_support_rfc9421 to match the new method name.

	public function test_rfc9421_is_unsupported() {

tests/includes/class-test-signature.php:687

• [nitpick] This test method name now mismatches the updated method. Consider renaming it to test_set_could_support_rfc9421 or similar to reflect the positive-support naming.

	public function test_set_rfc9421_unsupported() {