fix(opencode-usage): merge a migrated session's two rows per column (#22550)

A session that lived through OpenCode 2's V1 import has a row in both
`session` and `session_v2`, and neither is complete. #22391 resolved the
pair by ranking whole rows on one number — total token count, ties to
`session_v2` — which let that number decide everything else on the row.

Three consequences, each reproduced against that PR's own fixtures:

- A recorded cost could be zeroed. `session_v2` wins on tokens while
  carrying `cost = 0`, and row parsing maps a zero cost to `null`, so a
  legacy row's $12.50 disappeared. Cost is re-derived by the same lossy
  reduce as the tokens, but only the tokens were guarded.
- The token comparison decided metadata. A legacy row with more tokens
  supplied a stale pre-migration directory, and a legacy row with a NULL
  model erased the model `session_v2` had — 23 of 234 shared ids on a
  real migrated database have a model only on the v2 side.
- Winner-takes-all is per row, so a legacy row holding the input tokens
  and a v2 row holding the cache reads reported one of them as zero.

Metadata now comes from the generation OpenCode still writes, with older
generations filling only its NULLs; usage columns take a per-column MAX.
Both rows aggregate the same assistant messages, and the import can only
drop messages, never invent them, so each column's MAX is a tighter lower
bound on the truth than either row and can never exceed it.

The relation stays exactly one row per id — the highest-priority
generation holding it — every column stays `columnExists`-guarded with a
SQL fallback, and a database with a single session table builds the same
SQL it did before.

Cache schema version 4 -> 5 so existing caches rescan.

src/main/opencode-usage/opencode-usage-sqlite-fixture.ts

@@ -82,7 +82,8 @@ export type OpenCodeUsageFixtureSession = {
   id: string
   directory: string
   title?: string
-  model?: string
+  /** `null` writes a NULL `model`, the shape a v1 row has before the import derives one. */
+  model?: string | null
   cost?: number
   tokensInput?: number
   tokensOutput?: number
@@ -134,7 +135,9 @@ function insertSession(
     session.title ?? 'OpenCode session',
     created,
     session.timeUpdated ?? created + 60_000,
-    session.model ?? '{"providerID":"anthropic","modelID":"claude-sonnet-4-5"}'
+    session.model === undefined
+      ? '{"providerID":"anthropic","modelID":"claude-sonnet-4-5"}'
+      : session.model
   ]
   const usage = withUsageColumns
     ? [