[adenzhou1350]

TrioEventEmitter unconditionally awaits each handler's return value.
A successful synchronous handler therefore emits a spurious TypeError after
doing its work. A synchronous error handler can generate another error, and two
pending emits of an async once listener fail when the second wrapper correctly
returns None without invoking the listener again.

Call the handler as before, but await its result only when it is awaitable. The
existing exception-to-error dispatch remains in place; no thread offloading or
new scheduling policy is added.

Add regressions for synchronous on/once handlers, duplicate pending once
delivery, and a synchronous error handler. Update the changelog.

Validation on CPython 3.12.13 / Trio 0.34.0:

• Unchanged Windows source: all four new regression cases fail; the four existing
  Trio tests pass. Patched source: the complete 48-test suite passes, including
  asyncio, executor, Twisted, synchronous and uplift tests.
• Linux public-API probe: unchanged source fails three of five cases, while the
  patched source passes all five. This is not the full Linux pytest suite.
• Black/isort checks pass for the changed Python files; flake8 passes for
  pyee, tests and docs; git diff --check passes.

Other Python versions, Pyright and documentation builds were not run.
AI-assisted implementation and verification.

[jfhbrook]

I've gotten a number of AI assisted code contributions, and in light of that, I have a new checklist I'd like developers to run through:

• Has the developer read DEVELOPMENT.md?
• Have tests been updated accordingly?
• Have AI assisted commits been marked with a 🤖 emoji?
• Have AI assisted changes been marked with a 🤖 emoji in CHANGELOG.md?
• Does the PR notify the use of AI coding assistants?
• Is the developer's name in the CONTRIBUTORS.md file?

Some details on this are in DEVELOPMENT.md - I think it will help clarify any questions.

Anyway: If you can run through this checklist for me, I'd be much appreciative. Thanks!

[jfhbrook]

Also, if you are using AI, you may be interested in implementing AI skills/workflows and other agent/prompt files: #219

[jfhbrook]

Released in 15.0.0.