test: simplify historical migration fixtures

[Qiyuanqiii]

Summary

Store migration tests repeated database setup and wrapped an existing historical fixture with duplicate checks. Reuse openHistoricalTestDatabase throughout the affected tests, remove openHistoricalMigrationFixture, and replace hand-written prefix setup with independently copied historical databases. The v135 and legacy v138 tests now start at v134 and v136 directly.

Upgrade data, constraints, rejection and recovery assertions are preserved. Production migration SQL and the independent raw-migration oracles are unchanged. CONTRIBUTING now documents which fixture to use.

Based on #247 (which follows #246), so this diff contains only the migration-test cleanup.

Validation

• Focused internal/store validation on Windows with Go 1.26.6: all 128 selected top-level tests passed, covering affected upgrade tests, shared fixture callers and existing database-isolation contracts.
• The first batch reached Go's default 10-minute package timeout after 101 passes. Only the unfinished 27 were then run with -count=1 -timeout=15m; all passed in 251.228 seconds.
• git diff --check passed.

Surface governance

• No Surface is added, promoted, downgraded, deprecated, or removed.
• Registry item(s), tier, ownership, contracts, authority, platforms, compatibility and rollback: N/A — no Surface change.

Audit

• No credentials or local runtime data are included.
• Changes are limited to tests and contribution guidance; persistence and authority behavior is unchanged.