fix(tui): scope session.interrupt pending-prompt release to the calling session (#12441)

session.interrupt on session A was blast-resolving pending
clarify/sudo/secret prompts on ALL sessions sharing the same
tui_gateway process.  Other sessions' agent threads unblocked with
empty-string answers as if the user had cancelled — silent
cross-session corruption.

Root cause: _pending and _answers were globals keyed by random rid
with no record of the owning session.  _clear_pending() iterated
every entry, so the session.interrupt handler had no way to limit
the release to its own sid.

Fix:
- tui_gateway/server.py: _pending now maps rid to (sid, Event)
  tuples.  _clear_pending takes an optional sid argument and filters
  by owner_sid when provided.  session.interrupt passes the calling
  sid so unrelated sessions are untouched.  _clear_pending(None)
  remains the shutdown path for completeness.
- _block and _respond updated to pack/unpack the new tuple format.

Tests (tests/test_tui_gateway_server.py): 4 new cases.
- test_interrupt_only_clears_own_session_pending: two sessions with
  pending prompts, interrupting one must not release the other.
- test_interrupt_clears_multiple_own_pending: same-sid multi-prompt
  release works.
- test_clear_pending_without_sid_clears_all: shutdown path preserved.
- test_respond_unpacks_sid_tuple_correctly: _respond handles the
  tuple format.

Also updated tests/tui_gateway/test_protocol.py to use the new tuple
format for test_block_and_respond and test_clear_pending.

Live E2E against the live Python environment confirmed cross-session
isolation: interrupting sid_a released its own pending prompt without
touching sid_b's.  All 78 related tests pass.

tests/tui_gateway/test_protocol.py

@@ -120,7 +120,9 @@ def test_block_and_respond(capture):
 
     rid = next(iter(server._pending))
     server._answers[rid] = "my_answer"
-    server._pending[rid].set()
+    # _pending values are (sid, Event) tuples — unpack to set the Event
+    _, ev = server._pending[rid]
+    ev.set()
 
     threading.Event().wait(0.1)
     assert result[0] == "my_answer"
@@ -128,7 +130,8 @@ def test_block_and_respond(capture):
 
 def test_clear_pending(server):
     ev = threading.Event()
-    server._pending["r1"] = ev
+    # _pending values are (sid, Event) tuples
+    server._pending["r1"] = ("sid-x", ev)
     server._clear_pending()
 
     assert ev.is_set()