Add support for clearing field values with new "clear" operation

[Copilot]

This PR adds support for clearing field values in GitHub Projects using the new clear operation, which leverages GitHub's clearProjectV2ItemFieldValue GraphQL mutation.

Problem

Currently, there's no way to clear Date fields (or other field types) in GitHub Projects using this action. Users needed a way to remove values from fields, particularly Date fields, but the existing update operation only allows setting values, not clearing them.

Solution

Added a new clear operation that uses the ClearProjectV2ItemFieldValueInput GraphQL mutation to clear field values.

Key Changes:

• New clearField() function: Implements the clearProjectV2ItemFieldValue GraphQL mutation
• Enhanced input validation: Updated getInputs() to accept "clear" as a valid operation alongside "read" and "update"
• Extended main workflow: Enhanced run() function to handle clear operations with appropriate logging
• Comprehensive test coverage: Added 3 new tests covering input validation, field clearing, and end-to-end operation
• Updated documentation: Enhanced action.yml and README.md with clear operation details and usage examples

Usage Example:

- name: Clear due date
  uses: github/update-project-action@v3
  with:
    github_token: ${{ secrets.TOKEN }}
    organization: github
    project_number: 1234
    content_id: ${{ github.event.issue.node_id }}
    field: "Due Date"
    operation: clear

Backwards Compatibility

This change is fully backwards compatible. Existing workflows using read and update operations continue to work exactly as before. The clear operation is purely additive.

Testing

• All new functionality is covered by tests
• Existing tests continue to pass (4 pre-existing test failures are unrelated to this change)
• Code follows project linting and formatting standards

Fixes #144.

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[Copilot]

Pull Request Overview

This PR adds support for clearing field values in GitHub Projects by introducing a new "clear" operation that uses the clearProjectV2ItemFieldValue GraphQL mutation.

• Added a new clearField() function to execute field clearing operations
• Enhanced input validation to accept "clear" as a valid operation type
• Updated the main workflow to handle clear operations with appropriate logging and output

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

File	Description
src/update-project.ts	Implements the core clearField() function and integrates clear operation into input validation and main workflow
action.yml	Updates operation description to include "clear" option
test/main.test.ts	Adds comprehensive test coverage for clear operation input validation, function behavior, and end-to-end workflow
README.md	Updates documentation with clear operation usage examples and corrects parameter descriptions

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