fix(rust-client): restore bare-metal no_std builds

[huklaa]

Closes #2519.

Summary

• make the gRPC runtime dependencies optional behind the existing tonic feature
• generate message-only protobuf bindings when a no-default-features build does not enable transport support
• remove the remaining unconditional std-only dependency features from the bare-metal graph while preserving them under std
• require no-std consumers to provide an RNG explicitly through ClientBuilder::rng, since there is no platform entropy source
• add a thumbv7em-none-eabihf build target and CI job so bare-metal support cannot silently regress

Validation

• git diff --check passed
• Cargo feature-graph assertions passed for optional transport dependencies and restored std feature relationships
• the new make build-no-std command resolves to:
  cargo build --package miden-client --target thumbv7em-none-eabihf --no-default-features --locked
• the available local runner does not include Cargo, so the new hosted bare-metal job is the compile gate

Scope check

Immediately before submission, issue assignment/comments, open and recently merged PRs, related branches, and recent commits were checked. No overlapping implementation was found.