Fix vterm resize rendering for AI sessions

[cjf-666]

Avoid installing the height-only reflow advice for vterm, and route vterm dimension sync through its native resize handler. Flush queued vterm redraw output during resize instead of dropping it, so Codex-style TUIs keep their terminal state consistent without losing pending output.

Add regression coverage for vterm resize behavior and stale advice cleanup.

[Copilot]

Pull request overview

This PR improves terminal resize behavior for AI session buffers (especially Codex-style TUIs) by avoiding the previous “height-only reflow” advice path for vterm and instead delegating resizing to vterm’s native resize handler, while also ensuring any delayed vterm render output is flushed during resize to keep terminal state consistent.

Changes:

• Stop installing the reflow-filter advice for vterm resize handlers; keep it only for eat.
• Route vterm dimension syncing through vterm’s native resize handler and flush queued vterm render output during resize.
• Add regression tests covering vterm resize syncing, render-queue flushing, and stale reflow-advice cleanup.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ai-code-backends-infra.el	Adjusts resize handler lookup to accept an optional backend, disables reflow advice for vterm, and routes vterm dimension sync through vterm’s native resize handler (with render-queue flush).
ai-code-backends-infra-vterm.el	Adds a helper to immediately flush any queued/delayed vterm render output (used during resize).
test/test_ai-code-backends-infra.el	Updates/extends ERT coverage to ensure vterm no longer receives reflow advice, stale advice is removed, and vterm resize syncing flushes queued output.

[tninja]

@cjf-666 Thanks for the change. Will take a look

[tninja]

Change looks good!