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.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.