Add type checking for variadic function arguments

[AdamMagued]

Summary

Add argument type validation for variadic functions in functionEntry.resolveArgs.

Problem & Root Cause

In functionEntry.resolveArgs (functions.go), non-variadic functions validate both argument count and individual argument types via argSpec.typeCheck. However, when a function entry is marked variadic: true, resolveArgs only checked len(arguments) < len(e.arguments) and bypassed argSpec.typeCheck entirely.

As a result, passing non-object values (such as nil, strings, numbers, booleans, or arrays) into functions like merge() resulted in unhandled runtime panics during downcast type assertions inside function handlers (e.g. mapped := m.(map[string]interface{}) in jpfMerge in functions.go:519), rather than cleanly returning an evaluation error.

Fix

In functionEntry.resolveArgs, iterate over all supplied arguments for variadic functions and validate each argument using argSpec.typeCheck:

• Fixed prefix arguments (i < len(e.arguments)-1) are validated against e.arguments[i].
• Variadic arguments (i >= len(e.arguments)-1) are validated against e.arguments[len(e.arguments)-1].

If any argument fails type checking, an evaluation error is returned instead of panicking.

Verification

• Added TestVariadicFunctionTypeChecking covering merge() with invalid types (nil, string, number, array, boolean), merge() with missing/nil fields as reported in merge in go-jmespath / jp panics when parameters are not JSON objects #72, and not_null() with variadic arguments.
• Ran all existing unit and compliance tests (go test -v ./...), 100% passing.
• Checked code formatting (gofmt -l .) and static analysis (go vet ./...).

Fixes #72