Add readOnlyHint MCP tool annotations to Memgraph MCP server

[bbartels]

The Memgraph MCP server enforced read-only mode at runtime but never advertised readOnlyHint in tool metadata, leaving MCP clients unable to distinguish read-only tools from potentially mutating ones.

Changes

server.py — tool annotation updates

• 12 definitively read-only tools decorated with @mcp.tool(annotations={"readOnlyHint": True}):
  list_databases, get_configuration, get_index, get_constraint, get_schema, get_storage, get_triggers, get_procedures, get_betweenness_centrality, get_page_rank, get_node_neighborhood, search_node_vectors
• run_query uses readOnlyHint=READ_ONLY_MODE — the hint mirrors the runtime MCP_READ_ONLY env var so clients see accurate metadata in both modes:

  @mcp.tool(annotations={"readOnlyHint": READ_ONLY_MODE})
  def run_query(query: str, ctx: Context | None = None) -> list[dict[str, Any]]:

• use_database left unannotated — it mutates session state even though it doesn't write to the graph.

Uses FastMCP 3.x dict-style annotation API (annotations={"readOnlyHint": ...}), which maps directly to mcp.types.ToolAnnotations.

tests/test_tool_annotations.py — new test file

15 unit tests verifying:

• All 12 read-only tools expose readOnlyHint=True
• use_database does not have readOnlyHint=True
• run_query exposes readOnlyHint=True when MCP_READ_ONLY=true and readOnlyHint is not True when MCP_READ_ONLY=false

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[antejavor]

Hi @bbartels, thanks a lot for the feedback and proposed changes, just heads up, we are seriously refactoring current MCP setup -> #211

Tools are changing heavily, would you be wiling to adapt this PR once we merge #221 to main? Timing 😅

[bbartels]

@antejavor Oh I see, happy to! Alternatively feel free to just make this change there as well if it would be more convenient :)

[bbartels]

The main reason we need this is that it informs our mcp gateway whether or not a tool should be invokable from a readonly context!

[bbartels]

Also just a note, the cla requires a full address, feel like that might be a bit intrusive for some individuals