Add plugin tests for sticky payloads, validation, failures, and simulator behavior

[gravity-api]

Problem

SendUser32KeyboardKeyTests.cs (49 lines) contains only ManifestComplianceTest, NewPluginTest, and one behavior test covering just the no-driver no-op path — no test covers sticky-mode payload shape, Key/Delay validation, HTTP failure handling, or actual simulator-driven dispatch.

Proposed change

Add tests for: a sticky-mode request's payload shape, rejected invalid Key/Delay values (#79/#80), an HTTP failure surfacing as a reported error (post #81), and dispatch against a simulator driver (#86).

Compatibility requirements

1. Should land alongside the fixes each test covers (Reject negative and out-of-range User32 keyboard delays #79, Reject empty, null, malformed, and unknown keyboard key values #80, Report incompatible User32 drivers instead of silently succeeding #81, Add cancellation and bounded timeouts to User32 keyboard dispatch #86), not as a single deferred batch at the end.

Acceptance criteria

1. Each of the four scenarios has at least one passing test.