Fix RPC extension UI session reconciliation for editor/newSession flows

[jorgeavaldez]

Fixes RPC-mode extension UX so extension-driven flows (for example the handoff extension) behave like the pi CLI.

Handoff flow:

• generate a prompt
• let user edit it
• start a new session
• insert drafted prompt into the main input buffer

There were three gaps affecting extension flows:

1. missing support for extension_ui_request method editor
2. no explicit session-switch event when extensions call ctx.newSession()
3. session reconciliation after editor responses could fail when callbacks ran outside the chat buffer (e.g. temporary editor buffer context)

What changed

• Added full extension_ui_request editor support:

  • temporary editor replaces the existing input area (same window)
  • C-c C-c submit, C-c C-k cancel
  • restores previous buffer and cleans up temporary editor buffer on completion

• Fixed unsupported extension UI responses to always echo the original request :id (for correct RPC correlation).

• Added extension session reconciliation after extension-driven UI actions:

  • run get_state, compare previous/current session identity (session-file, fallback session-id)
  • if changed, reload get_messages history and update usage/header
  • otherwise refresh state/header only

• Expanded reconciliation triggers:

  • after sending extension UI responses (confirm / select / input / editor)
  • after set_editor_text
  • agent_end fallback sync for hook/tool-driven session changes

• Added handling for editor callbacks from non-chat buffers:

  • pi-coding-agent--extension-ui-send-response now accepts optional chat-buf
  • pi-coding-agent--extension-ui-sync-session-state now accepts optional proc/chat-buf
  • editor submit/cancel captures originating chat buffer and passes it through, so reconciliation runs against the correct session even when callback execution context is the temporary editor buffer

Validation

Added/updated unit tests for:

• editor submit/cancel behavior
• unsupported-method response correlation
• session sync changed vs unchanged paths after set_editor_text
• agent_end fallback sync
• extension UI response paths triggering sync
• editor submit sync when callback runs outside chat buffer
• handoff-style flow simulation: editor submit → new session state reconcile → set_editor_text

Render test suite passes when run against the local checkout.

Disclaimer: This pull request was put together with assistance from an LLM. I reviewed and revised it to the best of my abilities, and I do believe I have a solid understanding of how it works and why this is a good fix 😄