fix: keep attach button visible on a fresh thread

[91jaeminjo]

Problem

The composer's file-attachment button disappeared on a freshly created chat
thread in a room that supports attachments, even though room-scoped uploads
still worked.

Root cause

ThreadHistory.supportsAttachments was derived from the thread's replayed
AG-UI state. A thread with no finished run has no replayable state — and the
backend omits run_input for unfinished runs on the history endpoint — so it
resolved to a false negative that shadowed the authoritative room-level
capability.

Fix

• ThreadHistory.supportsAttachments is now tri-state (bool?): an empty
  AG-UI state means undetermined (null) rather than a definitive false.
• The room screen records a per-thread value only when it is resolved, so the
  attach gates fall back to room.supportsAttachments when undetermined. The
  thread-level signal still takes precedence when the thread has resolvable
  state.
• The three attach gates now share a single _attachEnabledForThread helper,
  so the undetermined→room fallback lives in one place.

Tests

• Domain: tri-state getter (true / false / null).
• Client API: getThreadHistory at the real wire shape (unfinished run,
  run_input: null) → supportsAttachments == null.
• Widget: undetermined state + skill room → attach shows; resolved non-sandbox
  state + skill room → attach hidden.

All room and client suites pass; analyzer clean.

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