fix(cli): fall back to PollSelector when kqueue can't register stdin (macOS 26+)

[evanstinger]

What does this PR do?

Fixes a crash on macOS 26+ where hermes (interactive chat mode) fails immediately with:

OSError: [Errno 22] Invalid argument
  ...
  selectors.py, line 523, in register
    self._selector.control([kev], 0, 0)

macOS 26 introduced a kqueue regression where registering stdin (fd 0) for EVENT_READ raises OSError [Errno 22] Invalid argument. prompt_toolkit triggers this via loop.add_reader(0, ...) in Vt100Input._attached_input().

Root Cause

Python's selectors.KqueueSelector (default on macOS) can no longer register fd 0 on macOS 26+. The existing workaround from #8560 (fstat guard + exception handler) doesn't prevent this crash because the OSError fires inside prompt_toolkit's context manager __enter__, before our exception handler is reached.

Fix

Probe kqueue early in startup (after _apply_profile_override(), before any prompt_toolkit import). If KqueueSelector can't register stdin, replace selectors.DefaultSelector with PollSelector, which works correctly.

Only activates on sys.platform == "darwin" when kqueue is actually broken — zero impact on other platforms or older macOS versions.

Related Issue

• Upstream: prompt-toolkit#1943
• Upstream PR (unmerged): prompt-toolkit#2065
• Related prior fix: fix: handle broken stdin in prompt_toolkit startup (#6393) #8560 (handles broken stdin from uv-managed Python, but not this kqueue regression)

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• hermes_cli/main.py: Add kqueue probe + PollSelector fallback after _apply_profile_override(), before any prompt_toolkit import

How to Test

1. On macOS 26+: Run hermes — previously crashed with OSError [Errno 22], now starts normally
2. On older macOS / Linux: No behavior change — kqueue probe passes, no fallback triggered
3. Verify the probe: python -c "import selectors; s=selectors.DefaultSelector(); s.register(0, selectors.EVENT_READ); s.unregister(0)" — fails on macOS 26+ with KqueueSelector, passes with PollSelector

Checklist

Code

• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've tested on my platform: macOS 26.4.1 (Tahoe, Apple Silicon M4 Pro)

Documentation & Housekeeping

• N/A — no config keys, architecture changes, or tool behavior changes

[teknium1]

Closing — superseded by #26077 (merged as commit d3d5916), which preventively probes kqueue at startup and falls back to SelectSelector when fd 0 cannot be registered. The widened except-clause matching EINVAL / EBADF / 'Invalid argument' — which most PRs in this cluster including yours added — is also included.

Thanks for the fix; closing as duplicate of the merged work.