test: add multi-language source-normalization fixtures

[EmersonBraun]

test: add multi-language source-normalization fixtures

Why this is useful

AgentsKit Code Review accepts many source languages, including Python and Go, but its checked-in review fixtures currently cover only TypeScript. A small multi-language regression fixture would make that support visible and protect the source-normalization contract without requiring a model or API key.

Scope

Add a focused fixture directory and extend the existing source tests:

• test/fixtures/sources/example.py — a minimal valid Python file;
• test/fixtures/sources/example.go — a minimal valid Go file;
• test/fixtures/sources/README.md — a non-code file that must be ignored; and
• test/sources.test.mjs — credential-free assertions for path and stdin normalization.

Acceptance criteria

• Calling loadTargets with the fixture directory returns exactly the Python and Go files.
• The returned targets report language as py and go, preserve the complete file contents, and set isChanged to false for path-based input.
• The Markdown fixture is ignored.
• One stdin assertion using a Python filename verifies language: "py" and isChanged: true.
• Assertions do not rely on filesystem iteration order.
• No provider, model, network request, generated snapshot, or production-code change is needed.

If the test exposes a production bug, please describe it in the PR and keep any fix narrowly scoped to agents/code-review/sources.ts rather than expanding the contribution.

Validation

npm run build
node --test test/sources.test.mjs
npm run check

Contribution boundary

Keep the change to the new synthetic fixtures and test/sources.test.mjs, unless the regression test demonstrates a small source-normalization bug. Do not add dependencies or call a hosted model.