fix: isolate review-mutation ledger from host session-state (#590)

Tests that call patch.dict(os.environ, clear=True) dropped
GITEA_MCP_SESSION_STATE_DIR and re-adopted the operator host cache
under ~/.cache/gitea-tools/session-state. A residual terminal
request_changes mutation then tripped the #332 hard-stop before
test_unknown_profile_blocks could assert the empty-profile gate.

- Pin mcp_session_state.default_state_dir / DEFAULT_STATE_DIR to a
  per-test temp dir in conftest (still honors an explicit env override)
- Clear the decision lock at the start of each TestMergePR test
- Add regression coverage for env-clear isolation

Closes #590

tests/conftest.py

@@ -1,4 +1,5 @@
 """Shared pytest fixtures for the Gitea-Tools test suite."""
+import os
 import sys
 
 import pytest
@@ -16,13 +17,46 @@ def _reset_mutation_authority(monkeypatch):
     the next test. It must never replace verify_mutation_authority with a
     no-op: individual tests that need a specific authority state set it up
     explicitly.
+
+    Session-state isolation (#559 / #590): many tests use
+    ``patch.dict(os.environ, ..., clear=True)``, which drops
+    ``GITEA_MCP_SESSION_STATE_DIR``. Relying only on the env var therefore
+    re-exposes the operator host cache
+    (``~/.cache/gitea-tools/session-state``) and its review-mutation ledger.
+    Pin ``default_state_dir`` / ``DEFAULT_STATE_DIR`` to a per-test temp dir
+    so durable load/save never touches host state, even after env clears.
     """
     monkeypatch.delenv("GITEA_SESSION_PROFILE_LOCK", raising=False)
     # Isolate durable session-state files so tests never share host cache (#559).
     import tempfile
 
     _state_tmp = tempfile.TemporaryDirectory(prefix="gitea-session-state-")
-    monkeypatch.setenv("GITEA_MCP_SESSION_STATE_DIR", _state_tmp.name)
+    state_dir = _state_tmp.name
+    monkeypatch.setenv("GITEA_MCP_SESSION_STATE_DIR", state_dir)
+    try:
+        import mcp_session_state
+    except Exception:
+        mcp_session_state = None
+    if mcp_session_state is not None:
+        # Survive patch.dict(..., clear=True) that drops the env var (#590).
+        # Still honor an explicit GITEA_MCP_SESSION_STATE_DIR when tests set
+        # their own temp dir (see tests/test_mcp_session_state.py).
+        monkeypatch.setattr(
+            mcp_session_state, "DEFAULT_STATE_DIR", state_dir, raising=False
+        )
+
+        def _isolated_default_state_dir(
+            _fallback: str = state_dir,
+            _env_key: str = mcp_session_state.STATE_DIR_ENV,
+        ) -> str:
+            raw = (os.environ.get(_env_key) or "").strip()
+            return raw or _fallback
+
+        monkeypatch.setattr(
+            mcp_session_state,
+            "default_state_dir",
+            _isolated_default_state_dir,
+        )
     try:
         import mcp_server
     except Exception: