feat(loader): support validate with standard schema

[ysknsid25]

attempts to resolve #228

[pi0]

@ysknsid25 i will put more time on this PR before releasing in next minor releases. It is a big feature!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.74%. Comparing base (42d9697) to head (1ebb06c).
⚠️ Report is 209 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   76.92%   77.74%   +0.82%     
==========================================
  Files           7        5       -2     
  Lines         806      364     -442     
  Branches       80      147      +67     
==========================================
- Hits          620      283     -337     
+ Misses        184       67     -117     
- Partials        2       14      +12     

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[ysknsid25]

Hello @pi0 . I have a consultation 🙇

Can you make feature branch for this issue?
I think I must writing test for all library complied Standard Schema, but there are 22 libraries.
In order to decrease your review workload, I think it would be better split sevral branch.

In this branch, write test for Zod and valibot and marge to new feature branch. remaning test code will be written another each branch.

How about? 👀

[pi0]

Hi dear @ysknsid25 i have updated changes little bit also simplified tests (to add again), fortunatly, since standard schema is unified, we only need to test against one library not all.

TODO:

• We need to infer return types when schema: { config } is provided
• We need better error formatting

[ysknsid25]

Hi dear @pi0 . Thank you for your helping :)

I changed code like below.

1. Return type inference from schema (src/types.ts + src/loader.ts)

Added function overloads to loadConfig so that when schema is provided, the return type of config is inferred from the schema's output type instead of falling back to Record<string, any>.

• With schema: Uses StandardSchemaV1.InferConfigOutput<S> to extract the config property type from the schema output
• Without schema: Behaves as before, using the explicit type parameter T

A new utility type InferConfigOutput was added to StandardSchemaV1 namespace to extract the config property from the schema output via conditional type inference.

2. Better error formatting (src/loader.ts)

Replaced raw JSON.stringify output with a human-readable format that combines each issue's path and message:

// Before 
[{"message":"Expected string, received number","path":["config","port"]}]

// After
Config validation failed:
  - config.port: Expected string, received number

3. Tests (test/loader.test.ts)

• Removed the now-unnecessary <UserConfig> type parameter from the existing test (inferred from schema)
• Added "schema validation formats errors" — verifies the error message starts with "Config validation failed:"
• Added "schema infers config type" — compile-time verification that config.configFile is correctly typed as string | boolean | undefined based on the schema.