feat: no missing external model provides fix

[benfdking]

• feat!: allow the linter to return fixes that affect other files #5057

• Note that at the moment it does not work when there is no external_models.yaml file. In this case it doesn't suggest the fix.

[Copilot]

Pull Request Overview

This PR implements an automatic fix feature for the "no missing external models" linter rule that adds missing external models to the external_models.yaml file. The fix intelligently handles whether the file ends with a newline or not to properly format the YAML addition.

• Adds automatic fix generation for missing external models in the linter
• Removes LSP-specific code for external model management (code lenses and commands)
• Adds comprehensive test coverage for the fix functionality with different file ending scenarios

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
sqlmesh/core/linter/rules/builtin.py	Implements create_fix method to automatically add missing external models to YAML file
tests/core/linter/test_builtin.py	Adds test cases for fix functionality with files ending/not ending in newlines
sqlmesh/lsp/main.py	Removes external model update commands and code lens features
sqlmesh/lsp/context.py	Removes LSP-specific external model update functionality