[ai] Create gemini guide file for the repository

PiperOrigin-RevId: 780214358

GEMINI.md

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+# gRPC C++ Gemini Collaboration Guide
+
+This document outlines conventions and best practices for AI-assisted development in the gRPC C++ codebase.
+
+## Preferred Tools & Libraries
+*   Prefer gRPC types before absl.
+*   Prefer std types when available, use absl types when not
+*   Prefer `std::optional` over `absl::optional`
+*   gRPC uses C++17, so we can't use C++20 onwards types.
+*   The Python implementation cannot depend on the protobuf library, so any shared libraries must expose a C-style API that does not rely on C++ protobuf types.
+
+## Code Style & Conventions
+*   `#include <grpc/support/port_platform.h>` is not required unless its macros are needed for compilation.
+*   Only include headers that are actively used.
+*   Abseil headers are sorted before gRPC headers.
+*   For public api headers (in `include/grpc`) we use `<grpc/...>`.
+    *   Example: `#include <grpc/grpc.h>`
+*   Prefer explicit types over `std::pair` or `std::tuple` for return types.
+*   Use `LOG(ERROR)` from `absl/log/log.h` for logging errors, never use `std::cerr` or `gpr_log`.
+*   The `fuzztest.h` header is located at `fuzztest/fuzztest.h`.
+
+## About gRPC
+*   gRPC uses its own macros for Bazel libraries, tests, etc. Each directory should have a `grpc_package` declaration. For libraries use `grpc_cc_library`, for tests `grpc_cc_test`.
+*   Dependencies on non-gRPC libraries (like gtest or absl) are listed in the `external_deps` attribute.
+*   The `:grpc` BUILD target is not allowed to depend on the C++ protobuf library, either directly or transitively.
+*   Build files for implementation code are typically located in `src/core/BUILD` and `BUILD`. Do not add new `BUILD` files under the `src/` tree without explicit instruction.
+*   Tests are located in `test/core` and `test/cpp` (corresponding to the `src` directories). These test directories contain their own `BUILD` files.
+*   Fuzz tests use `fuzztest_main` instead of `gtest_main`.
+*   When depending on a `grpc_proto_library`, the name of the `cc_library` target is the same as the `name` of the `grpc_proto_library` rule itself, not `[name]_cc_proto` as one might expect from standard Bazel `cc_proto_library` rules. The build system error messages can be misleading in this case.
+*   The 'gtest' external_dep also includes 'gmock'.
+*   All `upb` related build rules (`grpc_upb_proto_library`, `grpc_upb_proto_reflection_library`) for protos defined anywhere in the repository must be defined in the root `BUILD` file. They should not be placed in the `BUILD` file of the subdirectory where the proto is located.
+*   Core end-to-end tests are defined in `test/core/end2end/BUILD` using the `grpc_core_end2end_test_suite` macro. This macro generates multiple `grpc_cc_test` targets by combining a configuration file (like `end2end_http2_config.cc`) with individual test implementation files located in `test/core/end2end/tests/`. The final test target name is created by appending `_test` to the `name` attribute of the macro. For example, `grpc_core_end2end_test_suite(name = "end2end_http2", ...)` generates the test target `//test/core/end2end:end2end_http2_test`.
+*   Unused named parameters is a compilation failure.