test: E2E reproducer for remote proto module version bumps

[tinder-maxwellelliott]

What

Adds an E2E reproducer, testRemoteProtoVersionBumpImpactsConsumer, plus a hermetic proto_external_version_bump fixture, that validates a question we wanted answered directly: when a remote dependency that ships .proto files and exposes java_proto_library targets is version bumped, do the main-repo targets that consume those Java protos get invalidated?

Fixture shape

cli/src/test/resources/workspaces/proto_external_version_bump/:

• proto_dep — a module resolved via local_path_override (hermetic, but standing in for any remote/BCR module) that ships greeting.proto and exposes @proto_dep//:greeting_java_proto (loaded via @protobuf//bazel:java_proto_library.bzl, same as the repo itself).
• //:consumer — a main-repo java_library depending on @proto_dep//:greeting_java_proto.

What the test does

Copies the workspace twice and simulates a proto_dep 1.0.0 → 2.0.0 release the way a real version bump looks: it bumps the module version (root + dep MODULE.bazel) and adds a field to the shipped proto. It then runs generate-hashes (A vs B) with --fineGrainedHashExternalRepos @proto_dep and get-impacted-targets, asserting that both //:consumer and @proto_dep//:greeting_java_proto are reported impacted.

Result

bazel-diff correctly propagates the invalidation. Hand-verified with the locally built CLI — the bump impacts //:consumer, @proto_dep//:greeting.proto, @proto_dep//:greeting_proto, and @proto_dep//:greeting_java_proto, while the unchanged @proto_dep//:BUILD keeps its hash (genuine hash-change detection, not an artifact). So this lands as a positive regression lock, like the existing go_mod_change test — not a bug repro.

Notes

• The E2ETest.kt change is a clean addition (no reformatting of surrounding code); the file isn't ktfmt-formatted on master, so the addition matches the existing hand-formatted style.
• Includes a checked-in MODULE.bazel.lock (verified free of absolute-path leakage). E2E tests run on the Bazel 8.x/9.x bzlmod matrix, so no version-gating Assume was needed.
• Behavior holds only with --fineGrainedHashExternalRepos pointed at the proto repo; without it an external repo content change isn't picked up.

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