refactor: move MemgraphIngestor into services/graph behind a re-export shim and add the GraphIngestor protocol - #2

Summary

• Moves MemgraphIngestor from services/graph_service.py into a new services/graph/ package, leaving graph_service.py as a permanent re-export shim.
• Adds GraphIngestor, a protocol that extends the already-existing IngestorProtocol and QueryProtocol in services/__init__.py rather than inventing a parallel one.
• No behaviour change. MemgraphIngestor's body is byte-identical apart from the import depth.

First of a small series splitting up vitali87#1397, following the request there for smaller PRs. Each subsequent piece is independently green and goes out one at a time rather than stacked, so no PR ever shows a cumulative diff.

Type of Change

• Refactoring (no functional changes)

Related Issues

Related to vitali87#1397 — that PR should be closed or retargeted once this series lands.

Why

services/__init__.py already defines IngestorProtocol and QueryProtocol, and evals/cgr_graph.py's _CapturingIngestor already duck-types that surface. The seam exists informally today; this makes it explicit and gives the storage layer a package of its own instead of one 700-line module hanging off services/.

MemgraphIngestor is public API via cgr.__all__, so codebase_rag.services.graph_service has to keep resolving. The shim is deliberate and permanent, not a migration step.

Three patch() targets are repointed (test_graph_service.py, test_memory_limit.py) because they patch the module attribute, not the re-exported name.

A note on the diff size

GitHub renders this as a 691-line new file plus a 695-line deletion. With copy detection on it is a move plus 14 changed lines:

$ git diff -C -C --find-copies-harder --stat main..HEAD
 codebase_rag/services/graph/__init__.py            |   4 +
 .../{graph_service.py => graph/memgraph.py}        |  14 +-
 codebase_rag/services/graph/protocol.py            |  50 ++
 codebase_rag/services/graph_service.py             | 695 +--------------------
 codebase_rag/tests/test_graph_protocol.py          |  26 +
 codebase_rag/tests/test_graph_service.py           |   6 +-
 codebase_rag/tests/test_memory_limit.py            |   2 +-
 7 files changed, 98 insertions(+), 699 deletions(-)

The 14 lines in the moved file are all import-depth changes (.. to ...). protocol.py and the shim are the only genuinely new code.

Test Plan

• Unit tests pass — 7901 passed, 29 skipped
• New tests added — test_graph_protocol.py: the protocol is satisfied, the shim still exports the same object, cgr.__all__ still exposes it, and _CapturingIngestor still satisfies IngestorProtocol
• Integration tests pass — 300 passed, 5 skipped
• make pre-commit clean end to end

Checklist

• PR title follows Conventional Commits
• All pre-commit checks pass (make pre-commit)
• No hardcoded strings in non-config/non-constants files
• No # type: ignore, cast(), Any, or object type hints — none added
• No new comments or docstrings — not met, deliberately. protocol.py and the shim each carry one docstring, because "why does this indirection exist" is not derivable from the code: the shim in particular looks deletable without the note that it backs public API. This also reads as a conflict with CONTRIBUTING.md's comment policy ("explain why and how"). Easy to strip if the checklist is meant literally.