feat(test): add property-based testing with @fast-check/vitest (#266)

* chore(deps): add @fast-check/vitest for property-based testing

- Add @fast-check/vitest ^0.2.0 to dev catalog
- Enables native vitest integration for PBT

* test(tfidf-index): add property-based tests with @fast-check/vitest

- Verify scores are always within [0, 1] range
- Verify results are sorted by score in descending order
- Verify search returns at most k results
- Verify case-insensitive search produces consistent results
- Verify empty corpus always returns empty results
- Verify result IDs are from the indexed corpus
- Verify search is deterministic

* test(array): replace example-based tests with PBT

- Verify array input returns same reference
- Verify non-array non-nullish input wraps in single-element array
- Verify null/undefined returns empty array
- Verify result is always an array

Property-based tests cover all original cases and more edge cases.

* test(requestBuilder): add property-based tests for edge cases

- Test parameter key validation (valid/invalid characters)
- Test header management (accumulation, User-Agent, immutability)
- Test value serialization (strings, integers, booleans, arrays)
- Test deep object nesting within depth limits
- Test body type handling for all supported types

PBT improves coverage of boundary conditions and error paths.

* refactor(test): remove redundant tests now covered by PBT

- Remove depth limit test from requestBuilder (covered by PBT)
- Remove invalid key validation test (covered by PBT)
- Remove array serialization test (covered by PBT)
- Clean up tfidf-index tests: remove Score Validation, Edge Cases,
  Case Sensitivity, and Search Limits sections (all covered by PBT)

Coverage maintained at 92.45% statements, 93.47% lines.
Branch coverage improved from 83.98% to 84.43%.

* docs(test): add documentation comments to PBT explaining replaced tests

- Add JSDoc comments explaining what example-based tests were replaced
- Add inline comments with concrete examples for each property test
- Restore 'should handle arrays correctly within objects' test for clarity
- Keep concrete examples as documentation alongside PBT

Author: ryoppippi

package.json

@@ -59,6 +59,7 @@
 	"devDependencies": {
 		"@ai-sdk/provider": "catalog:dev",
 		"@ai-sdk/provider-utils": "catalog:dev",
+		"@fast-check/vitest": "catalog:dev",
 		"@hono/mcp": "catalog:dev",
 		"@types/node": "catalog:dev",
 		"@typescript/native-preview": "catalog:dev",

pnpm-lock.yaml

@@ -8,6 +8,7 @@ catalogs:
   dev:
     '@ai-sdk/openai': ^2.0.80
     '@ai-sdk/provider': ^2.0.0
+    '@fast-check/vitest': ^0.2.0
     '@ai-sdk/provider-utils': ^3.0.18
     '@clack/prompts': ^0.11.0
     '@hono/mcp': ^0.1.4