refactor(config.ts): handle undefined values and improve validation for OCO_OPENAI_MAX_TOKENS and OCO_MODEL (#176)

fix(config.ts): set default values for OCO_MODEL and OCO_LANGUAGE when reading from environment variables
refactor(generateCommitMessageFromGitDiff.ts): remove unused GenerateCommitMessageError interface

src/commands/commit.ts

@@ -1,6 +1,5 @@
 import { execa } from 'execa';
 import {
-  GenerateCommitMessageErrorEnum,
   generateCommitMessageByDiff
 } from '../generateCommitMessageFromGitDiff';
 import {