[Testing] Add automated tests for the generator, catch silent template breakage from schema changes

[Arjun544]

Overview

Currently there are no automated tests for the generator logic. A schema
change in app/lib/config/schema.ts can silently break template output
with no warning until someone manually runs the dev loop and notices.

This issue adds a test suite that validates the generator produces correct,
expected output for every schema configuration, catching breakage before
it reaches main.

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What Should Be Tested

1. Schema Validation Tests

• Every field in the schema has a valid default value
• Invalid config values are rejected with a clear error
• Adding a new field without a default fails loudly, not silently

2. Generator Output Tests

• For each architecture (Clean, MVVM, Feature-First), the generator
  produces the expected folder structure
• Expected files exist in the output (no missing files)
• No unexpected files are generated
• File contents match snapshots for critical files
  (e.g. main.dart, pubspec.yaml, router.dart)

3. Flag Combination Tests

• Each state management option (Riverpod, Bloc, Provider) generates
  the correct dependencies in pubspec.yaml
• Each routing option (GoRouter, AutoRoute) generates the correct
  router file
• Flags that depend on each other compose correctly
• Flags that conflict with each other are handled gracefully

4. Regression Tests

• A test is added for every bug fixed in the generator
• Schema changes that remove or rename a field fail existing tests
  immediately

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Implementation

Test Runner

The project already uses Vitest (vitest.config.ts exists). Add tests
under the existing tests/ directory.

Snapshot Testing

Use Vitest snapshot testing for generated file contents. This way any
unintentional output change is caught automatically and must be explicitly
approved before merging.

Test Structure

tests/
├── schema/
│ ├── validation.test.ts ← schema field and default tests
│ └── conflicts.test.ts ← incompatible flag combinations
├── generator/
│ ├── output-structure.test.ts ← folder and file existence
│ ├── snapshots/ ← Vitest snapshots of critical files
│ └── flag-combinations.test.ts
└── regression/
└── (one file per bug fixed)

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Acceptance Criteria

• bun test runs the full suite with zero manual steps
• Tests cover all three architectures
• Snapshot tests exist for at least main.dart, pubspec.yaml, router.dart
• CI workflow (see CI issue) runs bun test before dart analyze
• A schema change that breaks output causes at least one test to fail

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Notes

• Tests should run fast, mock the file system where possible rather than
  writing to disk
• Snapshot updates should be intentional: contributors must run
  bun test --update-snapshots explicitly and include the updated
  snapshots in their PR

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References

• tests/
• vitest.config.ts
• app/lib/config/schema.ts
• scripts/template-dev.ts
• docs/template-development.md