Add validation for config file extensions with clear error messages

[Copilot]

Plan: Add validation/warning for unsupported config file extensions

• Understand the current code structure and file validation approach
• Add file extension validation function in internal/config/viper.go
• Update SetFileName() in internal/config/config.go to validate extension before calling configureViper()
• Create appropriate localized error messages using localizer.Errorf()
• Add comprehensive tests in internal/config/viper_test.go for:
  • Valid extensions (.yaml, .yml)
  • No extension (for default sqlconfig file)
  • Invalid extensions (.txt, .json, etc.)
  • Edge cases (multiple dots, hidden files, files with only dot)
• Run tests to verify the implementation - all tests pass
• Manually test the error messages with invalid config files
• Address code review feedback (whitespace and import ordering)
• Run security scans - no issues found
• Address review comment: Validate extension BEFORE creating file
• Format error message as single line for consistency
• Add edge case tests for improved coverage
• Complete implementation

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add validation/warning for unsupported config file extensions</issue_title>
<issue_description>Description:
Currently, when users specify a config file with an unsupported extension (e.g., --sqlconfig myfile.txt), they receive a confusing Viper error: "Unsupported Config Type 'txt'".

Proposed Enhancement:
Add validation in internal/config/viper.go to check the file extension before passing to Viper and provide a clearer error message such as:

Error: Configuration files must use YAML format with .yaml or .yml extension.
The file 'myfile.txt' has an unsupported extension '.txt'.

Related:

• Issue Add documentation/help about supported config types #597 - Original documentation request
• PR Document that --sqlconfig flag only supports YAML format #684 - Documents YAML-only requirement

Implementation Notes:

• Check file extension in configureViper() or SetFileName() function
• Allow .yaml, .yml, and no extension (for default ~/.sqlcmd/sqlconfig)
• Provide helpful error message pointing users to documentation</issue_description>

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

• Fixes Add validation/warning for unsupported config file extensions #690

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[Copilot]

Pull request overview

This PR adds validation for configuration file extensions to provide clear, user-friendly error messages when users specify unsupported file extensions for the --sqlconfig flag. Previously, users would receive Viper's cryptic "Unsupported Config Type" error.

Changes:

• Added validateConfigFileExtension() function to check file extensions (.yaml, .yml, or no extension) in a case-insensitive manner
• Modified configureViper() to return an error instead of void, enabling proper error propagation
• Added comprehensive test coverage with 10 test cases covering valid extensions, invalid extensions, and edge cases

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
internal/config/viper.go	Implements validation logic for config file extensions and updates configureViper to return errors
internal/config/config.go	Updates SetFileName to handle errors from configureViper using checkErr
internal/config/viper_test.go	Adds comprehensive test coverage for extension validation including valid, invalid, and edge cases

[dlevy-msft-sql]

@copilot you've got review comments to fix

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[dlevy-msft-sql]

@copilot fix the review comment

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[dlevy-msft-sql]

@copilot fix the review issues

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.