Expand parity harness coverage before behavioral drift lands

The landed mock Anthropic harness now covers multi-tool turns, bash flows,
permission prompt approve/deny paths, and an external plugin tool path.
A machine-readable scenario manifest plus a diff/checklist runner keep the
new scenarios tied back to PARITY.md so future additions stay honest.

Constraint: Must build on the deterministic mock service and clean-environment CLI harness
Rejected: Add an MCP tool scenario now | current MCP tool surface is still stubbed, so plugin coverage is the real executable path
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep rust/mock_parity_scenarios.json, mock_parity_harness.rs, and PARITY.md refs in lockstep
Tested: cargo fmt --all
Tested: cargo clippy --workspace --all-targets -- -D warnings
Tested: cargo test --workspace
Tested: python3 rust/scripts/run_mock_parity_diff.py
Not-tested: Real MCP lifecycle handshakes; remote plugin marketplace install flows

rust/mock_parity_scenarios.json

@@ -0,0 +1,92 @@
+[
+  {
+    "name": "streaming_text",
+    "category": "baseline",
+    "description": "Validates streamed assistant text with no tool calls.",
+    "parity_refs": [
+      "Mock parity harness — milestone 1",
+      "Streaming response support validated by the mock parity harness"
+    ]
+  },
+  {
+    "name": "read_file_roundtrip",
+    "category": "file-tools",
+    "description": "Exercises read_file tool execution and final assistant synthesis.",
+    "parity_refs": [
+      "Mock parity harness — milestone 1",
+      "File tools — harness-validated flows"
+    ]
+  },
+  {
+    "name": "grep_chunk_assembly",
+    "category": "file-tools",
+    "description": "Validates grep_search partial JSON chunk assembly and follow-up synthesis.",
+    "parity_refs": [
+      "Mock parity harness — milestone 1",
+      "File tools — harness-validated flows"
+    ]
+  },
+  {
+    "name": "write_file_allowed",
+    "category": "file-tools",
+    "description": "Confirms workspace-write write_file success and filesystem side effects.",
+    "parity_refs": [
+      "Mock parity harness — milestone 1",
+      "File tools — harness-validated flows"
+    ]
+  },
+  {
+    "name": "write_file_denied",
+    "category": "permissions",
+    "description": "Confirms read-only mode blocks write_file with an error result.",
+    "parity_refs": [
+      "Mock parity harness — milestone 1",
+      "Permission enforcement across tool paths"
+    ]
+  },
+  {
+    "name": "multi_tool_turn_roundtrip",
+    "category": "multi-tool-turns",
+    "description": "Executes read_file and grep_search in the same assistant turn before the final reply.",
+    "parity_refs": [
+      "Mock parity harness — milestone 2 (behavioral expansion)",
+      "Multi-tool assistant turns"
+    ]
+  },
+  {
+    "name": "bash_stdout_roundtrip",
+    "category": "bash",
+    "description": "Validates bash execution and stdout roundtrip in danger-full-access mode.",
+    "parity_refs": [
+      "Mock parity harness — milestone 2 (behavioral expansion)",
+      "Bash tool — upstream has 18 submodules, Rust has 1:"
+    ]
+  },
+  {
+    "name": "bash_permission_prompt_approved",
+    "category": "permissions",
+    "description": "Exercises workspace-write to bash escalation with a positive approval response.",
+    "parity_refs": [
+      "Mock parity harness — milestone 2 (behavioral expansion)",
+      "Permission enforcement across tool paths"
+    ]
+  },
+  {
+    "name": "bash_permission_prompt_denied",
+    "category": "permissions",
+    "description": "Exercises workspace-write to bash escalation with a denied approval response.",
+    "parity_refs": [
+      "Mock parity harness — milestone 2 (behavioral expansion)",
+      "Permission enforcement across tool paths"
+    ]
+  },
+  {
+    "name": "plugin_tool_roundtrip",
+    "category": "plugin-paths",
+    "description": "Loads an external plugin tool and executes it through the runtime tool registry.",
+    "parity_refs": [
+      "Mock parity harness — milestone 2 (behavioral expansion)",
+      "Plugin tool execution path"
+    ]
+  }
+]