test(language/lint): add unit tests for unused-variable pass

[dsorajisto]

What

Adds 5 unit tests for the unused-variable lint pass in packages/language/src/lint/unused-variable.ts. Brings the rule's test count from 0 to 5.

Why

The unused-variable rule flags variables declared in the variables: block that are never referenced via @variables.<name> expressions, but had no tests pinning down its behavior. This closes the gap using the same pattern PRs #38 (required-fields.test.ts) and #48 (empty-block.test.ts) established for single-rule lint tests.

How

New test file at packages/language/src/lint/unused-variable.test.ts using vitest. Follows empty-block.test.ts exactly: single describe block, flat it cases, shared getDiagnostics helper. The test schema combines VariablesBlock (for variable declarations) with a small custom ValueBlock whose expr: ExpressionValue field provides somewhere for expressions to reference those variables.

The 5 cases:

1. Flags a variable that is declared but never referenced
2. Does not flag a variable that is referenced in an expression
3. Flags only the unused variables when multiple are declared
4. Reports one diagnostic per unused variable
5. Does not flag variables when no variables: block is declared

Test Plan

pnpm --filter @agentscript/language test unused-variable

Result:

✓ src/lint/unused-variable.test.ts (5 tests) 6ms
Test Files  1 passed (1)
     Tests  5 passed (5)

• pnpm --filter @agentscript/language test unused-variable — 5/5 passing
• npx eslint --config eslint.config.js packages/language/src/lint/unused-variable.test.ts — exit 0
• New/updated tests cover the change — N/A (this PR is the tests)

Checklist

• My code follows the project's coding style
• I have reviewed my own diff
• I have added/updated documentation as needed
• This change does not introduce new warnings