feat: add validation for file extensions in apply command

[NFUChen]

No description provided.

[greptile-apps]

Greptile Summary

This PR adds early file-extension validation to the apply command. The main changes are:

• Rejects JSON-looking paths passed to --file with a suggestion to use --plan.
• Rejects SQL-looking paths passed to --plan with a suggestion to use --file.
• Adds unit tests for case-insensitive extension checks and allowed file names.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
cmd/apply/apply.go	Adds CLI-side validation for likely --file and --plan flag mix-ups before apply execution.
cmd/apply/apply_test.go	Adds tests covering rejected .json schema inputs, rejected .sql plan inputs, and accepted valid extensions.

_{Reviews (1): Last reviewed commit: "feat: add validation for file extensions..." | Re-trigger Greptile}