fix(types): support fluent-json-schema

[salluexez]

Summary

Allow fluent-json-schema schemas to be passed to envSchema without TypeScript errors.

Problem

env-schema supports Fluent Schema objects at runtime and documents that usage, but EnvSchemaOpt.schema only accepted Ajv schema types. Consequently, valid Fluent Schema objects were rejected by TypeScript.

Solution

Add a dependency-free structural FluentSchema type and include it in the accepted schema union. This preserves existing Ajv and TypeBox support without adding a production or peer dependency.

Testing

• npm run lint
• npm test — 46 unit tests and 23 type assertions passed; 100% runtime coverage
• npm pack --dry-run --cache /tmp/env-schema-npm-cache
• git diff --check

Screenshots / Evidence

Not applicable; this is a TypeScript declaration fix.

Related Issue

Closes #138

Additional Notes

The regression test fails before the fix with TS2322 and passes afterward. There are no runtime changes or compatibility changes for existing schema types.

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[ilteoood]

• why it needs to change: fluent-json-schema's own README documents isFluentSchema as the public detection flag; isFluentJSONSchema is also set on every BaseSchema<T> but is not the documented public contract. The two-field structural type is broader than what the upstream docs promise to set.
• code suggestion:

  type FluentSchema = { isFluentSchema: boolean }

• why the suggestion differs: narrows the structural requirement to the single documented public flag; an object with isFluentSchema: boolean is a fluent schema per the upstream README, and the second field does not add coverage the documented contract guarantees.

[ilteoood]

• why it needs to change: this PR changes the types only. The runtime detection in index.js still uses Symbol.for('fluent-schema-object') + .valueOf(). The structural type is a type-level approximation of what the runtime already accepts, not a new contract, and a future reader of this line could infer that adding a field here changes runtime behavior.
• code suggestion: add a short // type-only: runtime uses Symbol.for('fluent-schema-object') in index.js above the union, or attach a JSDoc to the FluentSchema type pointing at the runtime path so the two stay in sync.
• why the suggestion differs: makes the type-only nature explicit at the point of use; prevents a future contributor from "tightening" the type to match a runtime field they think is checked and breaking valid fluent schemas.

[ilteoood]

• why it needs to change: this assertion uses S.object<EnvData>() to force the generic T, but fluent-json-schema does not infer T from .prop(...) arguments (unlike TypeBox). Users will not naturally type fluent schemas this way, and the test reads as promising more type inference than fluent-json-schema delivers.
• code suggestion: either drop this assertion (the optWithFluentSchema block at line 50 already proves the regression fix), or replace it with a .valueOf() assertion that locks in the documented escape hatch from Use with fluent-json-schema now requires to call valueOf in ObjectSchema when using Typescript #138, e.g. expect(S.object().valueOf as unknown as object).type.toBeAssignableTo<JSONSchemaType<EnvSchemaData>>(). If the generic-T path must be exercised, leave a one-line comment that it is a manual-typing witness, not runtime validation.
• why the suggestion differs: removes the over-claim about fluent-json-schema's type inference; the replacement locks in the workaround the original issue explicitly mentioned, and the regression is still proven by the first assertion.

[Puppo]

Hi @salluexez, thanks for your PR.
I'm not sure about this.
It seems like a Fluent workaround, not really an issue with this package.
In this branch, https://github.com/Puppo/env-schema/tree/test-fluent-schema, I prove that we should reach the same result without touching the codebase.

I'm happy to let you review your pr to add these tests if you'd like.
If you proceed, please also remember to add the test example to the JavaScript tests.

[Puppo]

Suggested change

	const optWithFluentSchema: EnvSchemaOpt = {
	schema: S.object().prop('PORT', S.number().default(3000).required()),
	}
	expect(optWithFluentSchema).type.toBe<EnvSchemaOpt>()
	const optWithFluentSchema = {
	schema: S.object().prop('PORT', S.number().default(3000).required()),
	}
	expect(optWithFluentSchema).type.toBeAssignableTo<EnvSchemaOpt>()

[Puppo]

Suggested change

	),
	).valueOf(),