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}

[Copilot]

Pull request overview

Adds early validation in the apply command to detect when a user passes a JSON file to --file or a SQL file to --plan, returning an actionable error suggesting the correct flag.

Changes:

• New validateFileExtension helper in cmd/apply/apply.go checks --file/--plan extensions (case-insensitive) and returns a helpful error on mismatch.
• RunApply invokes the new validator after the existing required-flag check.
• New unit tests cover both the validator directly and the end-to-end behavior via RunApply.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmd/apply/apply.go	Adds validateFileExtension and wires it into RunApply.
cmd/apply/apply_test.go	Adds table-driven and scenario tests for the new validation.

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[tianzhou]

LGTM. Thanks for the contribution.