Support case-insensitive string filters via static filtering

[MauruschatM]

Motivation

The adapter currently rejects any where clause with mode: "insensitive". Several Better Auth surfaces send them — e.g. the dashboard's user/organization search sends contains / starts_with clauses with mode: "insensitive" — and those queries throw against the Convex adapter today. This PR supports them and enables the upstream caseInsensitiveTestSuite.

Changes

• mode: "insensitive" is supported for eq/ne/in/not_in/contains/starts_with/ends_with: both sides are folded to lowercase and evaluated as static filters.
• Insensitive clauses are excluded from index selection, from the unique-field fast path, and from the in fan-out to point lookups — Convex index lookups are case-sensitive.
• Range operators (lt/lte/gt/gte) with insensitive mode still throw: case-insensitive ordering is ambiguous.

Performance note

An insensitive eq on a unique/indexed field (e.g. email) cannot use the index-backed lookup — it streams the table and filters statically, O(table size). For hot paths the existing recommendation stands: store values normalized (lowercased) on write and query case-sensitively. Happy to add a docs note if you can point me at where you'd want it — I didn't find an adapter-limitations section to update.

Tests

• Enabled the upstream caseInsensitiveTestSuite.
• Replaced the convex-custom test should reject case-insensitive where clauses with should reject case-insensitive range operators.

npm run build, npm run typecheck (src), and the full vitest suite pass: 10 files, 195 passed | 31 skipped (main baseline: 182 passed | 31 skipped).

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