feat(toolchain): support --incompatible_enable_proto_toolchain_resolution

[kangnak]

Fixes #402

Problem

//toolchain:compiled hardcodes @com_google_protobuf//:protoc. Because bzlmod repo mappings are per-module, a user who swaps @com_google_protobuf for a prebuilt protoc repo — e.g. with toolchains_protoc — only changes the mapping of their own root module. build_stack_rules_proto still resolves @com_google_protobuf to the protobuf module, so protoc is built from source. With the --per_file_copt guards recommended by toolchains_protoc, that surfaces as:

gcc: error: unrecognized command-line option '--PROTOBUF_WAS_NOT_SUPPOSED_TO_BE_BUILT'

Change

When --incompatible_enable_proto_toolchain_resolution is enabled, proto_compile now sources protoc from the registered proto toolchain (@rules_proto//proto:toolchain_type) — the same toolchain proto_library itself uses, and the one toolchains_protoc registers.

Resolution order:

1. the protoc rule attribute, if set
2. the proto toolchain, when the flag is enabled and one is registered
3. the //toolchain:protoc toolchain

With the flag disabled, behavior is unchanged: //toolchain:protoc remains a mandatory toolchain of the rule. With the flag enabled, both toolchain types are declared optional, so register_toolchains("@build_stack_rules_proto//toolchain:standard") is no longer required — and keeping it registered is harmless, since the proto toolchain takes precedence.

The compile action also passes the resolved FilesToRunProvider as a tool now (instead of just the executable File), so runfiles of a protoc wrapper are available to the action.

Files

• toolchain/toolchain.bzl — adds INCOMPATIBLE_ENABLE_PROTO_TOOLCHAIN_RESOLUTION, use_protoc_toolchains() and find_protoc().
• rules/proto_compile.bzl — uses the helpers. get_protoc_executable() is kept as a deprecated shim.
• docs/TOOLCHAINS.md — documents the flag and a toolchains_protoc setup.

Testing

A stub toolchain of type //toolchain:protoc whose protoc always exits non-zero was registered via --extra_toolchains, to prove which protoc is actually selected:

Configuration	Result
flag off	stub protoc used → build fails (legacy behavior preserved)
flag on	proto toolchain wins → build succeeds
flag on, proto toolchain type unregistered	falls back to //toolchain:protoc

Also verified:

• bazel build //... and bazel test //... pass (29/29, including //tools:buildifier.check)
• bazel build --incompatible_enable_proto_toolchain_resolution //example/... //google/... passes

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