fix(provider): assistant tool-call messages missing reasoning_content on Kimi K2.6 #23827

Problem
Using Kimi K2.6 with , long sessions fail with:

When it happens
Historical assistant messages that contain parts but no parts don't get injected. The existing fix () only covers messages after the last user turn.

Repro

1. Start a session with Kimi K2.6
2. Trigger a tool call (e.g. On branch fix/permission-reasoning-truncation
   Your branch is up to date with 'origin/fix/permission-reasoning-truncation'.

Untracked files:
(use "git add ..." to include in what will be committed)
fkyah3_dev/

nothing added to commit but untracked files present (use "git add" to track))
3. Continue conversation until ~80+ messages
4. Re-open the session and send any message → error on historical tool-call assistants

Fix approach
In , force empty for all assistant messages containing parts when is enabled, not just post-last-user messages.

Reference implementation
https://github.com/fkyah3/opencode-fkyah3/commit/d14da0b74

Tested on Win11 + Bun 1.3.11, OpenCode dev mode.