test: cleaned test suites and documented filtering guidelines

- Remove redundant definedness, null, and type checks across test suites in multiple packages.
- Clean up unused assertions, metadata tests, and obsolete test cases.
- Add good versus bad test filter guidelines and requirements to project documentation.

AGENTS.md

@@ -238,6 +238,21 @@ For the bash tool specifically:
 Test the contract the system exposes — not the easiest internal detail to assert.
 
 - Every new test must defend one **concrete, externally observable contract**: behavior, output shape, state transition, error mapping, or a regression-prone parsing boundary. If you cannot name the contract, do not add the test.
+
+### Good vs. bad test filter
+
+- **Name the failure mode.** Every test MUST state what a consumer observes if it regresses. Cannot name one? NEVER add it.
+- **Good: transformation.** One fixture MAY prove parse/render/normalize/encode/resolve behavior when output is computed, not echoed.
+- **Good: branch or boundary.** Distinct inputs, empty values, malformed input, version/provider routing, and state transitions MUST prove distinct outcomes.
+- **Good: external contract.** Exact bytes/shape MAY be asserted when a provider, parser, protocol, or persisted consumer reads them.
+- **Good: precedence or negative contract.** Keep explicit `false`/override-wins assertions and required absence only when they prevent a documented leak, downgrade, 400, or incompatible wire field.
+- **Good: regression.** A repro MUST trigger the prior real failure path and assert the corrected observable result.
+- **Bad: static echo.** NEVER test a constructor/builder merely copied a fixture or baked constant into an in-memory config/metadata field.
+- **Bad: success passthrough.** NEVER assert `fn(x) === x` when `x` was already supplied/declared valid; assert a transform, rejection, or downstream effect instead.
+- **Bad: wording/defaults.** NEVER assert prompt/UI boilerplate, a default literal, object existence, non-empty output, or length growth without a consumer contract.
+- **Bad: duplicate rows.** Parameterized/loop rows MUST each cover a distinct branch, provider/model path, or consumer contract; delete same-path duplicates.
+- **Metadata exception.** Exact metadata, identity, ordering, or `undefined` MAY remain only when a downstream consumer depends on it and the test establishes branch, precedence, negative-contract, wire, or regression evidence.
+- **Termination exception.** For cyclic/large inputs, assert a bounded output, surfaced error, or state change; bare `not.toThrow()` is insufficient.
 - No placeholder tests, tautologies, or "the code ran" assertions (`expect(true).toBe(true)`, bare `not.toThrow()`, non-empty string checks, length-grew checks, "prompt exists" checks without semantic assertion).
 - Prefer contract-level tests over implementation details. Avoid asserting internal helper wiring, field assignment, singleton identity, incidental ordering, prompt boilerplate, or passthrough option forwarding unless another component depends on that exact detail.
 - Don't duplicate coverage across abstraction levels. If an integration test already proves the behavior, drop the narrower unit test that restates it through mocks.