Neo4j backends: enforce read-only access (driver/session mode), all languages

[rahlk]

PROBLEM

The Neo4j backends are read-only in practice but not by construction. A sweep of cldk/ finds no MERGE, CREATE, execute_write, or write transaction anywhere — every backend goes through session.run(query, **params) and returns record.data(). But the driver is opened with GraphDatabase.driver(neo4j_uri, auth=...) (e.g. cldk/analysis/java/neo4j/neo4j_backend.py:113) and sessions are taken with self._driver.session(database=...), neither of which requests read access. Nothing stops a future write from silently succeeding against a production graph.

SCOPE BOUNDARY

Enforcement only. The original "separate ingestor" half of this issue is moot: there is no ingestion code in this SDK to separate out — graphs are populated outside it. This issue does not add an ingestor, and does not change any query.

GOALS

1. Open the driver with default_access_mode=READ_ACCESS (or take explicit read sessions) in all three backends: Java, Python, TypeScript.
2. A write attempted through a backend fails loudly rather than succeeding silently.

CAVEATS AND KNOWN RISKS

• Read access mode routes to followers in a cluster; against a single instance the behaviour is unchanged, so this is cheap, but on a cluster it changes which server serves the query.
• This is enforcement of an invariant that already holds. It should be a no-op for every existing test — if any test breaks, that test was writing through a read path and the breakage is the finding.

DEFINITION OF DONE

• All three Neo4j backends request read access explicitly.
• A test asserts that a write through a backend session raises rather than committing.
• The existing backend test suites pass unchanged.

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Re-scoped 2026-08-03: originally "read-only client + separate ingestor". The ingestor half was dropped after verification showed no write path exists in cldk/ at all — see the PROBLEM section for the evidence.