test(language/lint): add unit tests for expression-validation pass

[dsorajisto]

What

Adds unit tests for packages/language/src/lint/expression-validation.ts, which had no direct tests. 17 tests, following the same pattern as the empty-block (#48) and unreachable-code (#65) tests.

Coverage

• Function calls: built-ins accepted; unknown names flagged with a suggestion; arity violations (exact and variadic); method calls (namespace-function-call) and indirect calls (indirect-function-call)
• Namespaced functions: allowed call, unknown function in a known namespace, unknown namespace
• Operators: supported set, unsupported operator, custom supportedOperators
• Options: custom name-only functions set
• @system_variables.uploaded_files: bare dot and bracket references flagged; indexed, sliced and len() uses allowed; slicing @system_variables itself flagged

Each test also asserts that the input parsed with no syntax errors, so a grammar change can't make a "no diagnostic" test pass for the wrong reason.

Testing

pnpm vitest run in packages/language: 30 files, 324 tests pass.