Update grpcio and make gRPC generation portable

[caydyan]

What does this PR do?

Fixes #2171.

This updates the Python gRPC runtime/tooling to the current PyPI release and makes the generated-code import patching deterministic across environments.

Changes:

• update grpcio and grpcio-tools from 1.67.0 to 1.81.0
• make scripts/generate_grpc.sh fail fast with set -eu
• replace platform-specific sed -i import rewriting with a small Python rewrite step, so the generated *_pb2*.py files get package-relative imports consistently on Linux and BSD/macOS sed
• use printf for portable progress output

This is intentionally narrower than #2378: it only touches the dependency upgrade and gRPC generation path, without the unrelated API/lockfile changes that now conflict with main.

Verification

Passed locally:

• python3.12 -m venv .venv-grpc312
• .venv-grpc312/bin/python -m pip install grpcio==1.81.0 grpcio-tools==1.81.0 googleapis-common-protos==1.70.0
• PATH="$PWD/.venv-grpc312/bin:$PATH" sh scripts/generate_grpc.sh
• imported generated LND/CLN modules: hold_pb2, hold_pb2_grpc, lightning_pb2_grpc, node_pb2, node_pb2_grpc
• sh -n scripts/generate_grpc.sh
• .venv-grpc312/bin/python -m pip check
• git diff --check

I also attempted the full CLN docker-compose test with DOCKER_DEFAULT_PLATFORM=linux/amd64, but the local run could not complete because Docker Hub image pulls repeatedly failed with TLS handshake timeouts.

Bounty

This targets the ⚡Eligible for Sats ⚡ issue #2171. If this qualifies for the reward, payout address: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc.

[caydyan]

Quick review/routing check for this Eligible for Sats grpcio update.

Current contributor-side state:

• PR is open and non-draft.
• GitHub reports it as mergeable; REST reports mergeable_state=blocked, but I do not see any visible check-runs or status contexts on the head commit.
• The PR body includes the local verification that passed: fresh Python 3.12 venv, grpcio==1.81.0, grpcio-tools==1.81.0, portable scripts/generate_grpc.sh, generated LND/CLN imports, sh -n, pip check, and git diff --check.

The only validation I could not complete locally was the full CLN docker-compose run because Docker Hub pulls were failing with TLS handshake timeouts. If you need a specific CLN/LND reproduction command or a different proof before review, point me at the expected path and I can run it.

BTC payout address if this is accepted for the sats reward: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc