refactor(frontend): seal readonly leaks in app-types consumers (#1596)

[adamflagg]

Closes #1596.

Finishes the app-types readonly contract opened by #1595 — same latent readonly-leak class as the graph-type follow-ups #1593/#1594 (shipped in #1597). Two consumers were intentionally left out of #1595 to keep that a pure type-only change.

Changes

1. AllCampersView.tsx — drop the now-redundant as Camper[] cast. refactor(frontend): readonly on app-types cache/API shapes #1595 widened buildCamperRows to accept readonly Camper[] (csvExportHelpers.ts:73), and filteredCampers is MergedCamper[] (MergedCamper extends Camper), so it's directly assignable. The standing cast only suppressed future compiler signal if the contract changed.

2. mergeMultiSessionCampers.ts — mark AdditionalSession's fields and MergedCamper.additionalSessions readonly, matching the now-readonly Camper that MergedCamper extends. The builder constructs each AdditionalSession via .map() into fresh objects, so the element type going readonly is a clean change.

Pure type-only hardening — no runtime behavior change, no active mutation existed.

Tests

Adds the first tests for mergeMultiSessionCampers (previously untested):

• Behavioral coverage — multi-enrollment merge produces one entry with additionalSessions; single-session camper untouched.
• Readonly contract — @ts-expect-error assertions enforced by tsc --noEmit. Verified red→green: before the change the directives are unused (TS2578); after, they type-check.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Tests

  • Added comprehensive test suite for multi-session camper merging functionality, including validation of camper data consolidation and immutability constraints.

• Refactor

  • Enhanced type safety in CSV export and camper data handling to prevent potential bugs and ensure data integrity during export operations.

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ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 42e84ea9-c759-4469-8698-428c9f1f78f5

📥 Commits

Reviewing files that changed from the base of the PR and between 651453b and 839f549.

📒 Files selected for processing (3)

• frontend/src/components/AllCampersView.tsx
• frontend/src/utils/mergeMultiSessionCampers.test.ts
• frontend/src/utils/mergeMultiSessionCampers.ts

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📝 Walkthrough

Walkthrough

This PR completes the readonly immutability contract for MergedCamper and its AdditionalSession fields. Type contracts are tightened to enforce readonly semantics, comprehensive tests validate both runtime merge behavior and compile-time type safety, and a now-redundant type cast is removed from the CSV export path.

Changes

Readonly contract sealing

Layer / File(s)	Summary
Type contracts for immutable merged campers frontend/src/utils/mergeMultiSessionCampers.ts	AdditionalSession fields now marked readonly with explicit | undefined typing for optional properties. MergedCamper.additionalSessions updated from mutable to readonly AdditionalSession[] | undefined to seal the immutability hole in the readonly Camper base.
Test suite for merge behavior and readonly contract frontend/src/utils/mergeMultiSessionCampers.test.ts	Fixture builders and two behavioral tests validate multi-session collapse and single-session preservation. Type-level contract assertions using @ts-expect-error directives enforce readonly immutability at compile time via tsc --noEmit.
Remove redundant cast in CSV export frontend/src/components/AllCampersView.tsx	CSV export handler drops the as Camper[] cast, passing filteredCampers directly to buildCamperRows. Change is safe because MergedCamper[] is assignable to the widened readonly Camper[] contract.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Assessment against linked issues

Objective	Addressed	Explanation
Remove unnecessary as Camper[] cast in AllCampersView [#1596]	✅
Mark additionalSessions and AdditionalSession fields as readonly to match readonly Camper base [#1596]	✅
Validate readonly contract enforcement at compile time [#1596]	✅	@ts-expect-error assertions in test suite ensure TypeScript rejects attempts to mutate readonly fields.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/app-types-readonly-1596

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[adamflagg]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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