fix(session): tolerate pre-#24512 model JSON shape in fromRow

[kitlangton]

Closes #26435.

What broke

Sessions store their selected model as a JSON-typed SQLite column. On 2026-05-02, commit a3bc5d35b (PR #24512 "Refactor v2 session events as schemas" by @thdxr) renamed the JSON shape:

Before May 2	After May 2
{ "providerID": "opencode", "modelID": "big-pickle" }	{ "id": "big-pickle", "providerID": "opencode", "variant": "..." }

The PR updated:

• the Drizzle column annotation in session.sql.ts from $type<{ providerID, modelID }>() to $type<{ id, providerID, variant }>()
• fromRow in session.ts to read row.model.id

But $type<>() is a compile-time-only TypeScript cast. It does not transform, validate, or migrate the underlying JSON bytes. No data migration was added, so existing on-disk rows still contain { providerID, modelID }.

When fromRow reads one of those legacy rows it computes ModelID.make(row.model.id) — row.model.id is undefined, the Schema.brand("ModelID") decoder throws Expected string, got undefined, and the whole Session.list call rejects (no per-row containment in listByProject). One bad row hides every other valid session.

Why this slipped through review

• The TypeScript type changed; the storage shape didn't — the PR's diff looked self-consistent at the type level. CI tests use freshly-created rows, which already use the new shape, so they all pass.
• Read-only paths that fetch a session by ID (the common case during a normal chat session) don't necessarily read legacy rows.
• The crash only fires for users who: (a) had at least one session created before 2026-05-02, AND (b) ran opencode session list or anything else that hits listByProject. Many users create a fresh session each chat and never trigger list.
• The bug surfaced the moment someone with old rows ran the list command on Windows (session list crashes on legacy sessions with modelID key (expected id) #26435).

The fix

fromRow reads row.model.id ?? row.model.modelID so legacy rows decode using the old field name. Newly-written rows still use the new shape; this is read-side compatibility, not a write-side change. No data migration needed; no risk of corrupting existing data.

Reproducer

packages/opencode/test/session/session-legacy-model-shape.test.ts creates a session through the normal API, then directly mutates its model column to the legacy shape, then asserts Session.list returns it (and a sibling valid session) without crashing.

Verified red → green → red → green: pre-fix the test fails with the exact Expected string, got undefined error; post-fix it passes; reverting the fix makes it fail again.

Follow-up worth considering (not in this PR)

• Per-row containment in listByProject so a corrupt row never tanks the whole list, regardless of shape.
• A one-time migration that walks SessionTable and rewrites legacy model JSON to the new shape, eventually letting us drop this fallback.

[kitlangton]

Closing for further investigation. Review agents flagged factual issues with the PR's root-cause story, missing coverage in v2/session.ts:132, and an unsafe ?? "" fallback. Need to first establish how a real user ends up with {providerID, modelID} shape in the new model column — the column was added (not renamed) on 2026-05-02, so BennD's data must have come from a migration/import path or a non-mainline build. Will reopen with corrected story and fuller fix once that's clear.