[rust] Check in generated protobuf code and drop the protoc build requirement

[seokjin0414]

Fixes #661.

fluss_api.proto is static, but build.rs reran prost-build on every build, making a system protoc a requirement for every consumer of fluss-rs and for most workflows in this repo. This checks the generated code in, following the arrow-flight layout (committed prost output + a small gen crate + regen.sh):

• crates/fluss/src/proto/proto.rs: checked-in prost output, byte-identical to what build.rs produced (same prost-build version, same bytes config), with the ASF header and a do-not-edit notice prepended
• crates/fluss/gen: a tiny bin crate that regenerates it. prost-build is a normal dependency there, so the workspace compiles without protoc; protoc is only needed when actually running crates/fluss/regen.sh
• build.rs and the [build-dependencies] on prost-build are removed; cargo package -p fluss-rs --list confirms proto.rs ships in the crate and the nested gen crate is excluded automatically
• a new proto-up-to-date CI job reruns the regeneration and fails on any diff, so proto edits cannot land with stale generated code. The output is deterministic: prost-build is version-locked through Cargo.lock, and generation is byte-identical across protoc 27.1, 29.3 and 35.1
• the two Rust CI workflows stop installing protoc otherwise, so CI itself now proves the protoc-free build. The installs in the python/cpp/elixir/release workflows and the protoc gate in bindings/cpp/BUILD.bazel also become unnecessary, left for a follow-up to keep this reviewable
• DEVELOPMENT.md: protobuf is now only a prerequisite for regenerating

The first commit fixes the two clippy::for_kv_map errors that Rust 1.97 surfaced in accumulator.rs; they have kept check_license_and_formatting red on main since Jul 15, and this PR cannot go green without them.

Verified locally with no protoc installed: cargo build, cargo clippy --all-targets --workspace -- -D warnings, cargo test --all-targets --workspace with the CI excludes (572 passed), cargo fmt --all -- --check, cargo doc --workspace --no-deps --exclude fluss_python, and cargo check -p fluss-rs --features integration_tests --all-targets. Rerunning regen.sh reproduces the committed file byte-for-byte, and cargo deny check licenses plus the skywalking-eyes header check pass unchanged.

Context: apache/iggy#3688 (Fluss source connector for Apache Iggy; the protoc build requirement is the blocker there). If this lands, a 0.1.1 patch release would remove the protoc requirement for 0.1.0 consumers too.

[seokjin0414]

The Rust client moved into the apache/fluss monorepo, so this is re-targeted there: apache/fluss#3874. The monorepo version regenerates from the canonical FlussApi.proto (package fluss) instead of the old vendored subset, and drops the clippy commit since that is already fixed there. Closing.