fix: reject non-YAML extensions for --sqlconfig flag

[dlevy-msft-sql]

Problem

sqlcmd --sqlconfig config.json silently creates a JSON-named file but writes YAML content, leading to confusing behavior when users reference it later.

Fixes #690, Fixes #597

Root Cause

No extension validation in config.SetFileName().

Solution

• Validate file extension in SetFileName() before proceeding
• Reject extensions other than .yaml or .yml (extensionless files like the default ~/.sqlcmd/sqlconfig are still allowed)
• Report via checkErr (consistent with existing error patterns in this package)

Changes

File	Change
internal/config/config.go	Extension validation in SetFileName
internal/config/config_test.go	Tests for rejected and accepted extensions
README.md	Document custom config file extension requirement

Testing

• go build ./cmd/modern passes
• go test ./internal/config/... passes
• New TestSetFileName_RejectsNonYAMLExtension verifies .json/.toml rejected and .yaml/.yml accepted

[Copilot]

Pull request overview

This PR addresses confusing behavior where --sqlconfig accepts non-YAML extensions (e.g., .json) but still writes YAML content, by validating the config filename extension at internal/config.SetFileName() time.

Changes:

• Add extension validation in SetFileName() to reject non-.yaml/.yml extensions (while allowing extensionless filenames).
• Add unit test coverage for accepted/rejected extensions.
• Update README documentation for --sqlconfig custom configuration files.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
internal/config/config.go	Enforces YAML-only extensions (and rejects other extensions) for config filenames passed into the config subsystem.
internal/config/config_test.go	Adds coverage ensuring .json/.toml are rejected and .yaml/.yml are accepted.
README.md	Documents the expected YAML extension requirements for custom config files via --sqlconfig.

[Copilot]

Pull request overview

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