test(mcp): drive session re-attach and RAG attach filter guards through their real entry points

- use_session_applies_persisted_mcp_tools_immediately now loads a session
  file from disk through the real use_session, proving the post-assignment
  filter refresh applies a re-attached session's persisted allowlist.
- New use_rag_does_not_drop_role_filters loads a yaml-driver RAG through
  the real use_rag and proves the tool-scope rebuild recomputes the role's
  filter layer instead of dropping it.
- Seed the process-wide client/model registries in a pre-main ctor (new
  ctor dev-dependency) so model resolution is deterministic across test
  orderings; the seed exposes only an embedding model so tests that assert
  'no chat model available' keep their premise.

Cargo.toml

@@ -143,6 +143,7 @@ arboard = { version = "3.3.0", default-features = false }
 libc = "0.2"
 
 [dev-dependencies]
+ctor = "1.0.13"
 pretty_assertions = "1.4.0"
 rmcp = { version = "3.1.2", features = ["server"] }
 serial_test = "3"
@@ -154,4 +155,4 @@ path = "src/main.rs"
 [profile.release]
 lto = true
 strip = true
-opt-level = "z"
+opt-level = "z"