docs: state the tests-location rule in AGENTS.md §5.1

All test cases live in tests/<module>_tests.rs and access only public API;
internals are either tested through the public API or promoted to pub. The
sole exception is #[cfg(test)] test infrastructure (e.g. the permissive TLS
config in focus-transport).

AGENTS.md

@@ -243,6 +243,34 @@ focus-harness     ←  focus-core, focus-json
 | **集成测试** | `crates/<name>/tests/*.rs` | 跨模块的端到端（如 agent 循环跑完一轮） |
 | **mock 驱动** | `crates/focus-core/tests/agent_loop.rs` 内联 `MockProvider` | 用预设的 `StreamEvent` 序列驱动循环；仅当前单一测试文件使用时内联，多文件共享时抽为独立测试工具 crate |
 
+#### 5.1.1 测试位置硬性规则（所有 crate 统一遵守）
+
+- **所有测试用例**（单元 + 集成）都必须放在 `crates/<name>/tests/` 下，**禁止**在
+  `src/**` 里写 `#[cfg(test)] mod tests` 内联测试模块。`tests/` 下的文件是独立 crate，
+  只能访问被测 crate 的**公开 API**。
+- 当测试需要触及**内部实现**时，二选一：
+  1. **通过公开 API 测试**（优先）：如 provider 的请求体翻译用 mock transport
+     录制请求后断言其 JSON，而不是直接测私有 builder 函数；
+  2. **提升为 `pub`**：确有必要公开的内部组件（如 `ChunkedDecoder`、
+     `days_to_ymd`）提升为 `pub` 并写完整文档注释，测试放到 `tests/` 中。
+- **唯一例外**：`#[cfg(test)]` 的**测试基础设施**（非测试用例本身，如
+  `focus-transport/src/tls.rs` 中仅测试用的放行式 TLS 配置）允许留在 `src/`。
+
+#### 5.1.1 Rule: where tests must live (applies to every crate)
+
+- **All test cases** (unit + integration) must live under `crates/<name>/tests/`;
+  inline `#[cfg(test)] mod tests` inside `src/**` is **forbidden**. Files under
+  `tests/` are separate crates and can only see the tested crate's **public API**.
+- When a test must touch **internals**, pick one of:
+  1. **Test through the public API** (preferred): e.g. provider request-body
+     translation is asserted on the request recorded by a mock transport, not on
+     a private builder function;
+  2. **Promote to `pub`**: genuinely useful internals (e.g. `ChunkedDecoder`,
+     `days_to_ymd`) become `pub` with full doc comments, tested from `tests/`.
+- **Only exception**: `#[cfg(test)]` **test infrastructure** (not test cases —
+  e.g. the test-only permissive TLS config in `focus-transport/src/tls.rs`) may
+  stay in `src/`.
+
 ### 5.2 TDD 流程
 
 实现任何功能/修复前：