context-graph: message actions carry their plain text

[antejavor]

Closes #415. Part of map #390; the first ticket of the read path (#415 → #416 → #417 → #418).

What

• actions-graph: record_action writes a plain text property on user and assistant messages.
  • String content is stored as is; content blocks contribute their joined text blocks.
  • System messages, tool calls, tool results and empty messages get no text.
  • Turn text otherwise lives only inside the JSON string Action.properties, which Cypher can't unpack. The full-text index and the vector index from Where embeddings live, and what gets embedded #393 both need a plain property.
• Eval, text_search.ensure_turn_text_index: no longer reads content out in Python and writes a.text back. It counts the turns that carry text, creates the index, and runs a probe search.
• Eval, retrieval.py (graph-agent schema describer): text is never sampled. Messages now carry text from creation, so on a small graph, short turns looked enumerable and leaked into the prompt (test_schema_never_leaks_the_content_being_asked_about). Real eval graphs have long turns and never hit this. The frozen graph-agent baseline would see one more property in its schema.

Tests

All suites ran against a real Memgraph:

Suite	Result
actions-graph	55 passed; new: test_only_turns_carry_plain_text, test_an_empty_message_has_no_text
eval	280 passed, 5 skipped (API key)
sessions-graph	71 passed, 1 skipped