feat(discord): render clarify choices as buttons

Brings Discord to parity with Telegram on the clarify tool's interactive
UX. Overrides BasePlatformAdapter.send_clarify on DiscordAdapter to attach
a button view when choices are present.

  - ClarifyChoiceView: one discord.ui.Button per choice (max 24, Discord's
    25-component view cap leaves one slot for Other) plus a final
    'Other (type answer)' button.
  - Numeric click -> tools.clarify_gateway.resolve_gateway_clarify(
    clarify_id, choice_text) using the canonical choice text from the
    gateway entry (falls back to the button label if the entry vanished).
  - Other click -> tools.clarify_gateway.mark_awaiting_text(clarify_id) so
    the gateway's text-intercept captures the next user message in this
    session as the response.
  - Auth via the shared _component_check_auth helper (same OR-semantics as
    ExecApprovalView / SlashConfirmView / UpdatePromptView / ModelPickerView).
  - Open-ended (no choices) path renders the prompt as a plain embed and
    relies on the existing text-intercept resolution.
  - Single-use: first valid click disables every button and updates the
    embed footer with who answered and what they chose.

No changes to BasePlatformAdapter.send_clarify or the gateway's
clarify_callback wiring -- the existing scaffolding already drives all
adapters; Discord just inherits the default text fallback today and gains
buttons by virtue of this override.

Test conftest extended: _FakeEmbed gains add_field() / set_footer() stubs
so tests can construct embedded views without monkey-patching per-test.

Original PR: #19249 by @LeonSGP43. This is a reshape of the contributor's
work onto current main's clarify infrastructure (clarify_id + entry-based
resolution shared with Telegram, instead of a parallel on_answer-closure
mechanism). The button view structure and UX shape are preserved.

Tests: 14 new tests in tests/gateway/test_discord_clarify_buttons.py.
391/391 existing Discord gateway tests still pass.

Co-authored-by: LeonSGP43 <cine.dreamer.one@gmail.com>

tests/gateway/conftest.py

@@ -119,6 +119,14 @@ def _ensure_discord_mock() -> None:
             self.title = title
             self.description = description
             self.color = color
+            self.fields = []
+            self.footer = None
+        def add_field(self, *, name=None, value=None, inline=False, **_):
+            self.fields.append({"name": name, "value": value, "inline": inline})
+            return self
+        def set_footer(self, *, text=None, icon_url=None, **_):
+            self.footer = {"text": text, "icon_url": icon_url}
+            return self
     discord_mod.Embed = _FakeEmbed
 
     # ui.View / ui.Select / ui.Button: real classes (not MagicMock) so