EF UserTasks safe search: title/summary/reference/taskType still collation-cased (Tags fixed in #8111)

Problem

After #8111 / PR #8146, EF tag search is explicitly case-insensitive (Tags.Any(t => t.ToLower().Contains(tag))) to match InMemory/VNext. Title, summary, reference, and task type still use raw Contains(value), which follows the database collation.

InMemory and VNext use StringComparison.OrdinalIgnoreCase for every safe-search field. On case-sensitive providers (notably PostgreSQL default), search=approve can miss title Approve invoice on EF while matching on InMemory/VNext — portable worklist search is not equivalent.

Evidence

• Contract / FR-032: safe search over title, summary, reference, tags, task type (case-insensitive in reference InMemory)
• InMemory: InMemoryUserTaskRepository.MatchesSearch — all fields OrdinalIgnoreCase
• VNext: VNextUserTaskRepository.Matches — same ignore-case for those fields + tags
• EF: EFCoreUserTaskRepository.ApplySafeSearch — Tags use ToLower(); Title/Summary/Reference/TaskType use Contains(value) only
• Conformance SafeSearchByTagReturnsOnlyTheMatchingTask asserts tag case fold (PRIORITY-ESCALATION) but does not assert title/summary case fold
• Note: REST already documents that title sort/cursors are collation-nonportable; that does not excuse search semantic drift

Proposal

1. Make EF title/summary/reference/taskType search case-insensitive in a provider-portable way (same spirit as the Tags ToLower path — e.g. compare lowercased columns/expressions, or EF.Functions.ILike where already used elsewhere in the repo). Prefer the smallest change that matches InMemory/VNext substring semantics.
2. Extend conformance: title Approve invoice, search=approve and search=APPROVE both return the task on InMemory, Sqlite EF, and VNext (and keep existing tag cases).

Notes for triage

Milestone left unset for Elsa 3 Issue Triage / Elsa 3 Crew Lead. Soft proposal: 3.9 Backlog — Triage/Crew Lead decide (leftover from #8111 Tags fix; not a re-file of Tags omission).