test(template): add type compiler validation tests (JhaSourav07#3311)

## Description

Fixes JhaSourav07#2872 

Added `app/template.type-compiler.test.tsx` to provide isolated type
compiler validation and schema constraint stability coverage for
`app/template.tsx`.

Implemented 5 tests covering:
* Type enforcement for the `children` prop using `expectTypeOf`
* Validation that invalid prop parameters are blocked during static type
checking
* Optional/valid ReactNode compatibility checks
* Runtime rendering validation for valid children
* Stability verification for `null` children without runtime crashes

Verified the new test file passes with:
`npx vitest run app/template.type-compiler.test.tsx`

## Pillar
* [ ] 🎨 Pillar 1 — New Theme Design
* [ ] 📐 Pillar 2 — Geometric SVG Improvement
* [ ] 🕐 Pillar 3 — Timezone Logic Optimization
* [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
N/A (test-only change)

## Checklist before requesting a review:
* [x] I have read the `CONTRIBUTING.md` file.
* [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
* [ ] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
* [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
* [ ] I have updated `README.md` if I added a new theme or URL
parameter.
* [x] I have started the repo.
* [x] I have made sure that i have only one commit to merge in this PR.
* [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
* [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07