Fallback when async prompt admission fails

[crankshift]

Summary

Add a safe fallback when OpenCode async prompt admission is unavailable or rejected, so Telegram prompts keep working on servers that do not support /prompt_async.

Context

After switching long prompt handling to async prompt admission, some running OpenCode servers may reject the async admission endpoint immediately. In that case the gateway should fall back to the existing synchronous prompt request rather than failing the Telegram update immediately.

Acceptance Criteria

• Async prompt admission errors for unsupported or incompatible /prompt_async calls fall back to synchronous session.prompt.
• Completion timeout and assistant message failures still fail normally after successful async admission.
• A regression test covers the fallback path.
• The changelog documents the fallback behavior.

Notes

This is a follow-up to issue #53 / PR #54, which was merged before the fallback commit was included.