refactor(spider-proto-rust): Move protobuf codegen out of build.rs into a dedicated build binary and task

[coderabbitai]

Context

During review of #333, an offline discussion concluded that running protobuf code generation inside Cargo's build.rs is not the ideal approach.

Proposed Solution

• Introduce a dedicated build binary (e.g. a Cargo binary crate) responsible for protobuf code generation, rather than relying on build.rs.
• Add a dedicated task (in the taskfiles) to invoke this binary and control when codegen runs.
• The generated code is written to a stable destination: components/spider-proto-rust/src/generated.
• Use the task runner's checksum feature to determine whether codegen needs to be re-executed (i.e. only rerun when proto source files change).

Benefits

• Codegen is fully decoupled from Cargo's build lifecycle.
• Task checksums provide reliable, explicit caching.
• Simpler and more transparent build process.

References

• Introduced in feat(huntsman): Add protobuf scaffolding and gRPC StorageClient for task instance management: #333
• Requested by @LinZhihao-723