feat: lint rule no missing external will create file

[benfdking]

Summary

• extend linter Fix to support creating files
• add CreateFile dataclass
• enhance NoMissingExternalModels rule to create external_models.yaml if missing
• surface file creation edits in LSP code actions
• test linter and LSP behavior for file creation

Testing

• pytest tests/core/linter/test_builtin.py::test_no_missing_external_models -q
• pytest tests/lsp/test_code_actions.py::test_code_actions_create_file -q
• pytest tests/core/linter/test_builtin.py::test_no_missing_external_models_with_existing_file_ending_in_newline -q
• pytest tests/core/linter/test_builtin.py::test_no_missing_external_models_with_existing_file_not_ending_in_newline -q
• pytest tests/lsp/test_code_actions.py::test_code_actions_with_linting -q

---

https://chatgpt.com/codex/tasks/task_e_688b3aa3749883309ac7292ddb5440cd

[Copilot]

Pull Request Overview

This PR extends the linter's Fix mechanism to support file creation in addition to text edits. The main purpose is to enable automatic creation of missing configuration files when linting detects missing dependencies.

• Adds CreateFile dataclass to support file creation operations
• Enhances the NoMissingExternalModels rule to automatically create external_models.yaml when missing
• Integrates file creation capabilities into LSP code actions for seamless editor support

Reviewed Changes

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

Show a summary per file

File	Description
sqlmesh/core/linter/rule.py	Adds CreateFile dataclass and extends Fix to include file creation operations
sqlmesh/core/linter/rules/builtin.py	Updates NoMissingExternalModels rule to create external models file when missing
sqlmesh/lsp/context.py	Enhances LSP code actions to handle file creation operations
tests/core/linter/test_builtin.py	Tests the new file creation functionality in the linter
tests/lsp/test_code_actions.py	Tests LSP integration for file creation code actions