test: unit tests for stdlib components (react, unit_test_eval)

[planetf1]

Add unit tests for stdlib components and frameworks with zero coverage.

Scope

File	Stmts	What's missing	Est. tests
`stdlib/components/unit_test_eval.py`	71	JSON loading, Pydantic schema validation, `set_judge_context()` formatting — all pure logic, used by the eval CLI	~10
`stdlib/components/react.py`	33	`ReactInitiator` (goal + tool list formatting, finalizer injection), `ReactThought` — pure component logic	~5
`stdlib/frameworks/react.py`	39	Loop termination on `final_answer`, budget exhaustion, tool response collection — testable with mocked backend	~6

Notes

• `unit_test_eval.py` is the easiest win — pure Pydantic + JSON, no LLM needed, used in production by `cli/eval/runner.py`
• The react components are pure formatting logic, straightforward to test
• `frameworks/react.py` needs a mocked `Backend.aact()` but the core loop logic (termination, budget, tool dispatch) is well-defined
• Part of test: improve test coverage across formatters and core #813

~21 tests across the three files.