Add Auth sheet validation for empty/whitespace-only fields

[Copilot]

Prevents authentication failures by validating required Auth sheet fields before processing. Previously, empty or whitespace-only values in CLIENT_ID, CLIENT_SECRET, REDIRECT_URI, or CODE_VERIFIER would cause cryptic OAuth errors downstream.

Changes

• Added isBlank() utility: Validates string fields, handling empty strings, whitespace, null, and undefined values
• Validation in generateAuthUrl(): Checks CLIENT_ID and REDIRECT_URI before generating auth URL
• Validation in doGet(): Checks CLIENT_ID, CLIENT_SECRET, CODE_VERIFIER, and REDIRECT_URI before token exchange
• New error constant MSG_AUTH_FIELD_EMPTY: Provides consistent field-specific error messages
• Updated README.md: Added new error message to the message list

Example

// Before: OAuth fails with cryptic Microsoft API error
// After: Clear error at validation point
if (isBlank(clientId)) {
    throw new Error("Client IDが空またはスペースのみです。正しい値を入力してください。");
}

Validation occurs before any external API calls, providing immediate feedback when Auth sheet configuration is incomplete.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[機能要望] Authシートのバリデーションを強化したい</issue_title>
<issue_description>## 概要
Authシートのバリデーションを強化したいです。</issue_description>

Comments on the Issue (you are @copilot in this section)

@Shinoryo ## 認証URL生成処理 CELL_CLIENT_ID、CELL_REDIRECT_URIの値が空文字列・空白のみである場合、認証URL生成せず、エラーとする。

doGet時

CELL_CLIENT_ID、CELL_CLIENT_SECRET、CELL_CODE_VERIFIER、CELL_REDIRECT_URIが空文字列・空白のみである場合、トークン取得せず、エラーとする。</comment_new>

• Fixes [機能要望] Authシートのバリデーションを強化したい #28

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[Shinoryo]

確認しました。

[Copilot]

Pull request overview

This PR adds validation for authentication sheet fields to prevent OAuth failures caused by empty or whitespace-only values. The changes provide early feedback when configuration is incomplete, replacing cryptic downstream API errors with clear, actionable messages.

Key Changes:

• Added isBlank() utility function to validate string fields for empty, whitespace-only, null, or undefined values
• Implemented validation in generateAuthUrl() for CLIENT_ID and REDIRECT_URI before auth URL generation
• Implemented validation in doGet() for CLIENT_ID, CLIENT_SECRET, CODE_VERIFIER, and REDIRECT_URI before token exchange
• Added MSG_AUTH_FIELD_EMPTY constant for consistent field-specific error messages

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
MicrosoftToDoImporter.gs	Added isBlank() utility function, validation logic in generateAuthUrl() and doGet(), and MSG_AUTH_FIELD_EMPTY constant
README.md	Added documentation for the new error message (line 166) describing when validation errors occur

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