New chat creation fails with AbsolutePathBuf deserialized without a base path — reproduces across all trusted workspaces (Windows VS Code extension 26.513.21555)

Environment

VS Code extension: openai.chatgpt version 26.513.21555 (openai.chatgpt-26.513.21555-win32-x64)
OS: Windows 11
Auth: ChatGPT account (Business plan)
Codex CLI: also installed via npm (@openai/codex), latest

Reproduction

Open VS Code in any trusted workspace listed in ~/.codex/config.toml
Click the Codex panel
Type any prompt (e.g., hello) and press Enter

Expected
A new chat session opens and Codex responds.
Actual
Red error banner: Error starting chat — Invalid request: AbsolutePathBuf deserialized without a base path. The attempt appears in the task list with a warning icon but never opens.
Observations

Continuing an existing task works correctly and returns valid GPT-5.5 responses — auth, network, model routing, plugins are all healthy. The bug is isolated to new-chat creation.
Reproduces across multiple distinct workspaces (AI-Office-Manager, HR-Planning-Hub-Prod, more). Not project-specific.
An empty no-workspace window gives a different and expected message: "Add a project to use Codex". So the bug triggers specifically when a trusted project context is loaded during new-chat init.

Already ruled out

No [agents.*] blocks in ~/.codex/config.toml (so not #19257 / #19261)
No ~/.codex/agents/ folder
No project-level .codex/config.toml
Not WSL agent mode (Windows native, sandbox = "unelevated") — so not #16815
Stripped \\?\ Windows long-path prefixes from [marketplaces.*] source lines → no effect, restored
Full cache wipe of ~/.codex/.tmp and ~/.cache/codex-runtimes (Codex re-fetched cleanly) → no effect
Uninstall + reinstall both Claude Code and Codex extensions → no effect
Sign out + sign back in → no effect
Model 5.5 High → 5.5 Medium → no effect
Auto-review → Default permissions → no effect
Only one VS Code window open → no effect

Hypothesis
Looks like another AbsolutePathBuf deserialization site missing an AbsolutePathBufGuard, similar to #19257 / #19261 but on the new-chat session creation path rather than the agent role config path.