feat: add transaction() context manager to CoordinodeClient

[polaz]

Feature

Add explicit transaction support to the Python SDK once CoordiNode implements Bolt-level transaction lifecycle (BEGIN / COMMIT / ROLLBACK).

API design

Context manager (preferred):

with client.transaction() as tx:
    tx.cypher("CREATE (n:Person {name: $name})", params={"name": "Alice"})
    tx.cypher("CREATE (n:Person {name: $name})", params={"name": "Bob"})
    # auto-commit on clean exit, auto-rollback on exception

Explicit API:

tx = client.begin_transaction()
try:
    tx.cypher("MERGE (n:Entity {name: $name})", params={"name": "Alice"})
    tx.commit()
except Exception:
    tx.rollback()
    raise

Implementation notes

• Wire to Bolt BEGIN / COMMIT / ROLLBACK messages once R508 is available
• CoordinodeClient.transaction() returns a Transaction object that wraps a Bolt session
• Transaction.cypher() sends queries within the open transaction (no auto-commit per statement)
• The existing client.cypher() stays as auto-commit shorthand for single statements
• LlamaIndex and LangChain adapters: evaluate wrapping upsert_nodes + upsert_relations calls in a transaction for atomicity

Gate

Blocked by CoordiNode DB task R508 — Bolt protocol transaction lifecycle (BEGIN / COMMIT / ROLLBACK).
This is Phase 5.1. Do not start SDK implementation until CoordiNode adds Bolt transaction support.

Acceptance criteria

• client.transaction() context manager works correctly
• Transaction.cypher() sends queries in-transaction
• Rollback on exception is automatic
• Integration test: verify partial write is rolled back on error
• client.begin_transaction() / tx.commit() / tx.rollback() explicit API
• Documented in README