fix(cluster): build a NodeInfo before calling NodeRegistry.register in RegisterNode

[AmirF194]

What

ClusterServiceImpl.RegisterNode (the gRPC entry point for bernstein.v1.ClusterService/RegisterNode) raised TypeError on every call.

Why

NodeRegistry.register(node: NodeInfo) -> NodeInfo takes a single positional NodeInfo. The gRPC handler instead called it with NodeInfo's own constructor keywords:

node = self._registry.register(
    name=request.name,
    url=request.url,
    capacity=cap,
    labels=dict(request.labels) if request.labels else {},
    cell_ids=list(request.cell_ids) if request.cell_ids else [],
)

TypeError: NodeRegistry.register() got an unexpected keyword argument 'name', unconditionally, for any node trying to join a cluster over gRPC. The two other call sites in the codebase (POST /cluster/nodes in task_cluster.py, and ClusterAuthGate.register_node in cluster_auth.py) already build a NodeInfo first and pass it positionally; RegisterNode was the one call site that skipped that step.

How

Build the NodeInfo from the request fields, then pass it positionally, matching the two working call sites.

Fixes #3500

Verification

• New regression test (TestClusterServiceImplRegisterNode::test_register_node_stores_a_real_node) fails on unmodified main with the exact TypeError above, and passes on this branch, asserting the registered node's name, url, capacity fields, labels and cell_ids.
• uv run python scripts/run_tests.py -k grpc (13 tests) and -k cluster (13 files, 172 tests) pass. Did not run the full suite.
• ruff check src/, ruff format --check, and lint-imports pass; pyright on the changed file goes from 183 to 179 pre-existing errors (this file sits outside pyrightconfig.strict.json's scope; the drop is the removed invalid-kwargs call).
• Not verified: the gRPC wire path end to end (no cluster_pb2 stubs generated in this environment). The test calls ClusterServiceImpl.RegisterNode directly with a fake request and a mocked cluster_pb2 module.

Checklist

• uv run ruff check src/ passes
• uv run pyright src/ passes (pre-existing repo-wide backlog outside pyrightconfig.strict.json, unrelated to this change)
• uv run python scripts/run_tests.py -x passes (ran only -k grpc and -k cluster, see Verification)
• New code has type hints

Documentation duty (every PR that touches a feature)

• User-visible README section updated (or N/A if internal-only) - N/A, no user-facing behavior beyond the bug fix, noted in the changelog
• docs/operations/<area>.md updated (or N/A) - N/A
• docs/api/ schema regenerated if a public surface changed (or N/A) - N/A, no proto/schema change
• uv run bernstein agents-md sync run so AGENTS.md, CLAUDE.md, .goosehints, CONVENTIONS.md, and .cursor/rules/*.mdc reflect any new module (or N/A) - N/A, no new module
• Tests cover the documented behaviour