refactor: dedup setSessionId calls in session-switch path; add test coverage

Code review of 893f049 flagged 6 findings. This commit addresses them all:

#1 (real issue, production code): setSessionId was being called
TWICE per session switch - once explicitly from the cell-action
callback, and again from the prompt's _handleSwitchSession (which
also assigned _sessionId, called _resetStreamPointers, and set
_sessionSelect.value directly). The explicit setSessionId in the
cell-action callbacks (onCreateSession, onSwitchSession) is now
removed; the prompt's handlers do all of that bookkeeping through
a single setSessionId() call, which already implements the
dropdown + reset-stream-pointers logic.

#2 / #4 (test coverage): the existing switching test only checked
that the right HTTP call was made. New assertions: the prompt's
session-messages mock IS called again (history actually reloaded)
and the rendered history reflects the new session's content
(verifies the user sees the right messages, not stale DOM).

#3 (test coverage): new test for the error path. bind_existing
returns ok:false -> the prompt rolls the dropdown back to the
previous active id, leaves _sessionId unchanged, and shows a
Notification.error. This locks in the rollback behavior so a
future refactor can't silently break it.

#5 (test cleanup): mockedCallOpenCodeSessionMessages (the spy used
to verify history reload) is now declared at the top of the spec
file alongside the other mocked* variables, with a meaningful
name (mockedSessionMessages) instead of an inline jest.mocked()
that was hard to find.

#6 (source-doc): added a TODO(frontend) comment on
callOpenCodeSetActiveSession explaining why it is currently
unused (kept for a future 'switch between already-bound sessions'
flow that would avoid the bind round-trip) and warning a future
maintainer not to delete the corresponding server route without
removing the client function (or vice versa).

Tests: 77 jest (was 76), 73 pytest, build green.

Co-Authored-By: Claude <noreply@anthropic.com>

src/components/opencode_inline_prompt.ts

@@ -491,9 +491,9 @@ export class OpenCodeInlinePrompt extends Widget {
     }
     try {
       const newId = await this._options.onCreateSession();
-      this._sessionId = newId;
-      // Drop any in-progress streaming DOM (the new session is empty).
-      this._resetStreamPointers();
+      // setSessionId() handles _sessionId + dropdown + reset pointers
+      // in one call; avoids duplicate work in this handler.
+      this.setSessionId(newId);
       this._historyEl.textContent = '';
       // Re-fetch the global list so the new id is in the dropdown
       // and marked active.
@@ -529,8 +529,9 @@ export class OpenCodeInlinePrompt extends Widget {
     try {
       const ok = await this._options.onSwitchSession(sessionId);
       if (ok) {
-        this._sessionId = sessionId;
-        this._resetStreamPointers();
+        // setSessionId() handles _sessionId + dropdown + reset
+        // pointers in one call; avoids duplicate work.
+        this.setSessionId(sessionId);
         this._historyEl.textContent = '';
         if (this._options.onReloadHistory) {
           try {
@@ -539,10 +540,6 @@ export class OpenCodeInlinePrompt extends Widget {
             console.warn('opencode_bridge: reload history failed', e);
           }
         }
-        // Re-mark the now-active option as selected.
-        if (this._sessionSelect) {
-          this._sessionSelect.value = sessionId;
-        }
       } else {
         // Roll back the select to the previous value.
         if (this._sessionSelect) {