chore - Bump MSRV/toolchain to stable 1.93 and fix strict clippy

[mingley]

Summary

This PR bumps the repo to Rust 1.93.0 and applies the minimal, behavior-preserving fixes required to keep strict linting clean.

Detailed, file-by-file changes:

1. Cargo.toml

• Added rust-version = "1.93.0" under [package].
• Why: declare the current minimum supported Rust version explicitly.

2. rust-toolchain.toml

• Changed toolchain channel from 1.84.1 to 1.93.0.
• Why: align pinned toolchain with the declared rust-version.

3. src/common/errors.rs

• Changed Error::GrpcAPI payload from tonic::Status to Box<tonic::Status>.
• Added explicit impl From<tonic::Status> for Error that boxes the status.
• Why: remove the need for broad clippy::result_large_err allowances while preserving existing conversion ergonomics for ?.

4. src/store/request.rs

• Updated .map_err(Error::GrpcAPI) to .map_err(Error::from).
• Why: match the boxed GrpcAPI constructor while keeping behavior unchanged.

5. src/proto.rs

• Added module-level #[allow(dead_code)] on mod protos.
• Added explicit context comments:
  • Rust 1.93 clippy::all now flags many protobuf-generated wire types as dead code.
  • Prior 1.84.1 toolchain did not fail on these generated definitions.
• Why: keep the suppression narrowly scoped to generated proto modules and document why it is needed now.

6. src/pd/retry.rs

• Simplified map(|resp| resp.stores.into_iter().map(Into::into).collect()) to map(|resp| resp.stores).
• Why: remove a redundant conversion flagged by stricter clippy.

7. src/transaction/requests.rs

• Replaced iter::repeat(...).take(len) with iter::repeat_n(..., len).
• Simplified self.keys = shard.into_iter().map(Into::into).collect(); to self.keys = shard;.
• Removed redundant .map(Into::into) when merging lock responses.
• Why: eliminate manual_repeat_n and useless_conversion lints without changing runtime behavior.

8. src/transaction/transaction.rs

• Simplified .map(|r| r.into_iter().map(Into::into).collect()) to .map(|r| r.into_iter().collect()) in both affected execution paths.
• Why: remove redundant conversion paths flagged by clippy.

9. tests/integration_tests.rs

• Replaced iter::repeat(vec![]..).take(batch_num) with iter::repeat_n(vec![].., batch_num).
• Why: fix manual_repeat_n lint in tests under stricter clippy.

Testing Done

Executed locally on branch mingley/bump-rust-version-1-93:

1. make check

• Includes:
  • cargo run -p tikv-client-proto-build
  • cargo check --all --all-targets --features "integration-tests"
  • cargo fmt -- --check
  • cargo clippy --all-targets --features "integration-tests" -- -D clippy::all
• Result: pass.

2. cargo nextest run --config-file $(pwd)/config/nextest.toml --all --no-default-features

• Result: pass.
• Summary: 53 passed, 0 skipped.

Follow-on PR Callout: Dependency Modernization

This PR intentionally limits scope to toolchain + lint compatibility. A dedicated follow-on PR should upgrade stale dependencies in controlled waves.

Major gaps from cargo +1.93.0 outdated -R --depth 1:

• tonic: 0.10.2 -> 0.14.3
• prost: 0.12.6 -> 0.14.3
• thiserror: 1.0.69 -> 2.0.18
• rand: 0.8.5 -> 0.9.2
• prometheus: 0.13.4 -> 0.14.0
• async-recursion: 0.3.2 -> 1.1.1
• Dev/tooling: clap 2.34.0 -> 4.5.57, env_logger 0.10.2 -> 0.11.8, proptest-derive 0.5.1 -> 0.8.0, rstest 0.18.2 -> 0.26.1, serial_test 0.5.1 -> 3.3.1, reqwest 0.11.27 -> 0.13.1

Why this matters:

• Security/maintenance exposure increases on stale major versions.
• Ecosystem compatibility with modern tonic/prost APIs is deferred debt.
• Duplicate dependency trees (notably multiple HTTP/rand stacks) increase compile and maintenance cost.

Recommended follow-on sequence:

1. Upgrade transport stack (tonic/prost) in isolation.
2. Upgrade core error/runtime deps (thiserror, rand, async-recursion, prometheus).
3. Upgrade dev/test/tooling deps.
4. Keep each wave green with make check + nextest before moving forward.

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[mingley]

Addressed local review findings #2 and #3 in cfad559:

• Removed crate-wide/example clippy::result_large_err suppressions.
• Replaced Error::GrpcAPI(tonic::Status) with boxed storage + explicit From<tonic::Status> so API signatures remain unchanged while reducing error size pressure.
• Dropped generated-file rewrite logic from proto-build/src/main.rs.
• Moved dead-code suppression to generated module boundary in src/proto.rs with explicit “why now vs before” context comment.

Also added a dedicated "Follow-on PR Callout: Dependency Modernization" section to the PR description with major version gaps and rationale.