test(mcp): add HTTP transport integration tests covering every endpoint

29 integration tests in crates/axon-server/tests/mcp_contract.rs exercise
the full MCP JSON-RPC stack against an in-process server backed by
SQLite in-memory storage — same router as production, no mocks.

Coverage:
  Protocol:  ping, initialize (capabilities + serverInfo), notifications
             (→ 204), unknown method (-32601), malformed JSON (-32700),
             session header issued on first request
  tools/list: axon.query always present; CRUD tools appear after collection
  tools/call: item.create, item.get (missing → isError), item.patch
              (success + version conflict → isError), item.delete,
              unknown tool → isError, item.aggregate count
  resources/list: _collections and _schemas always present; axon://item
                  appears after collection created
  resources/templates/list: entity-by-id and audit templates present
  resources/read: axon://_collections, axon://item (listing), axon://item/{id}
                  (entity), missing entity → JSON-RPC error (-32602)
  resources/subscribe / unsubscribe: subscriptionId returned; unsubscribe ok
  prompts/list: all four known prompts present
  prompts/get:  axon.schema_review returns user message; unknown → error
  SSE endpoint: GET /mcp/sse delivers text/event-stream over real HTTP transport

Verification: cargo test -p axon-server --test mcp_contract,
cargo clippy -p axon-server --tests -- -D warnings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: easel