Expose AI Assist custom system prompts

[adamtheturtle]

Closes #14.

What changed

• expose Question.aiAssistCustomSystemPrompt from ai_assist_custom_system_prompt
• support the field in question create and update payloads without breaking existing initializers
• preserve the value through optimistic question copies
• teach the mock server to create and update the field, including updates to newly created questions
• cover present, null, absent, create, and update cases

Why

Question-specific AI Assist instructions were returned by the API but discarded by CoderPadKit, preventing faithful review and synchronization of AI-enabled questions.

Validation

• swift test — 119 tests passed

CI note

The macOS build/test check passes. The SwiftLint job currently fails on 35 pre-existing violations in unrelated Screen files on the base branch; none are in this PR’s changed files.

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Note

Low Risk
Additive optional field and mock merge fix; no auth or breaking API changes to existing initializers.

Overview
Adds Question.aiAssistCustomSystemPrompt (ai_assist_custom_system_prompt) so interviewer-only AI Assist instructions from the API are decoded, sent on create/update, and kept on optimistic question copies instead of being dropped.

Mock server: create responses persist the field; allQuestions() now merges updates onto session-created questions (not only fixtures) so PUT round-trips work for new IDs.

Tests cover decode (present / null / absent), mock create+update, and spacing preservation.

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