fix(channel): read reactions from the reactions endpoint

[mikield]

getReactions() bound the wrong endpoint constant:

Endpoint::bind(
    Endpoint::CHANNEL_MESSAGES,   // channels/:channel_id  — one slot
    $channelId,
    $messageId,
    (string) $emoji,              // ← both dropped on the floor
)

The bound URL is channels/:channel_id/messages. So the call fetched the channel's most recent messages and mapped them into User objects. No error, no empty result — a plausible-looking list of users that has nothing to do with who reacted.

getReactions binds: channels/CH/messages
should be:          channels/CH/messages/MSG/reactions/EMOJI

Why the test did not catch it

HttpHelperTestCase mocks Http::get() with no argument expectations and asserts only the return type. Nothing in the REST suite checks which endpoint a method actually called, so a method can bind any constant at all and still pass.

The harness now captures the requested URL and asserts it when a case supplies one:

'validationOptions' => [
    'returnType' => User::class,
    'array' => true,
    'url' => 'channels/::channel id::/messages/::message id::/reactions/%3A%3Aid%3A%3A',
]

Opt-in, so this PR stays about the bug, but it is worth backfilling across the suite — this is the third method in this repo found to have never worked, and the first two were also invisible for want of an assertion someone assumed was there.

Found while porting a bot that reads back petition votes on start-up to catch up on what it missed while it was down.

🤖 Generated with Claude Code

[github-actions]

🎉 This PR is included in version 1.3.3 🎉

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