Expand test coverage and remove low-value tests

[kylebrowning]

Stacked on #246; will be retargeted to main once it merges.

• First-ever wire tests for liveactivity (update + start), pushtotalk, widgets, and location through a real client; priority-5 and no-expiration header checks.
• Alert encode variants: localized title/body keys, .fileName sound, all interruption levels.
• Direct unit tests for APNSRequest.headers (the URLSession client's entire header contract) and APNSEnvironment/APNSBroadcastEnvironment URL composition.
• withBadge/withSound tests now actually decode and assert the payload; testRequestID asserts the echoed apns-request-id.
• Removes tests that couldn't fail meaningfully (channel-list encode, two token-window tests subsumed by the boundary test); fixes a test-name typo and tearDown hygiene.