TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 4: refactor(mcp_client): migrate 8 Batch B sites to Result[T]

Phase 4 Batch B (8 INTERNAL_BROAD_CATCH sites in mcp_client.py):

Added _result variants inside the Result Variants region:
  - get_git_diff_result (subprocess.run + CalledProcessError)
  - ts_c_get_skeleton_result (ASTParser.get_skeleton)
  - ts_c_get_code_outline_result (ASTParser.get_code_outline)
  - ts_c_get_definition_result (ASTParser.get_definition)
  - ts_c_get_signature_result (ASTParser.get_signature)
  - ts_c_update_definition_result (ASTParser.update_definition)
  - ts_cpp_get_skeleton_result (ASTParser.get_skeleton with lang=cpp)
  - ts_cpp_get_code_outline_result (ASTParser.get_code_outline with lang=cpp)

Plus 5 internal _ast_* helpers (extract ASTParser boilerplate).

Each legacy string-returning function now delegates to its _result variant;
the try/except Exception is REMOVED from the legacy function.

Updated test_baseline_result.py:
  - Phase 3 tests loosened (BC<=32, total MIG<=80)
  - Phase 4 tests added (BC=24, total MIG=72, modules import cleanly)

Audit: mcp_client BC 32 -> 24. Total MIG 80 -> 72.

tests/test_baseline_result.py

@@ -119,21 +119,27 @@ def test_phase2_per_file_baseline_counts_match_inventory():
 # ============ Phase 3 tests (3) ============
 
 def test_phase3_mcp_client_broad_catch_decreased_from_40_to_32():
+    """Phase 3 Batch A migrated 8 INTERNAL_BROAD_CATCH sites.
+    This test asserts the snapshot AFTER Phase 3 (BC=32).
+    Subsequent phases will loosen this test (it documents the Phase 3 boundary).
+    """
     data = _audit_live()
     files = {f["filename"]: f for f in data["files"]}
     findings = files["src\\mcp_client.py"]["findings"]
     bc = sum(1 for f in findings if f["category"] == "INTERNAL_BROAD_CATCH")
-    assert bc == 32, f"expected mcp_client BC=32 after Batch A, got {bc}"
+    assert bc <= 32, f"expected mcp_client BC<=32 after Phase 3, got {bc}"
 
 
 def test_phase3_total_migration_target_decreased_to_80():
+    """Total MIG was 88; after Phase 3 it's <=80 (Batch A migrated 8).
+    Subsequent phases will loosen this test."""
     data = _audit_live()
     files = {f["filename"]: f for f in data["files"]}
     total = 0
     for key in TARGETS:
         findings = files[key]["findings"]
         total += sum(1 for f in findings if f["category"] in MIG)
-    assert total == 80, f"expected total MIG=80 after Phase 3, got {total}"
+    assert total <= 80, f"expected total MIG<=80 after Phase 3, got {total}"
 
 
 def test_phase3_audit_baseline_matches_phase1_audit_json():
@@ -143,4 +149,39 @@ def test_phase3_audit_baseline_matches_phase1_audit_json():
     for key in TARGETS:
         findings = files[key]["findings"]
         total += sum(1 for f in findings if f["category"] in MIG)
-    assert total == 88, f"PHASE1_AUDIT_BASELINE.json expected 88 baseline MIG, got {total}"
+    assert total == 88, f"PHASE1_AUDIT_BASELINE.json expected 88 baseline MIG, got {total}"
+
+
+# ============ Phase 4 tests (3) ============
+
+def test_phase4_mcp_client_broad_catch_decreased_to_24():
+    """Phase 4 Batch B migrated 8 more BC sites (32 -> 24)."""
+    data = _audit_live()
+    files = {f["filename"]: f for f in data["files"]}
+    findings = files["src\\mcp_client.py"]["findings"]
+    bc = sum(1 for f in findings if f["category"] == "INTERNAL_BROAD_CATCH")
+    assert bc == 24, f"expected mcp_client BC=24 after Phase 4, got {bc}"
+
+
+def test_phase4_total_migration_target_decreased_to_72():
+    """Total MIG was 88; after Phase 4 Batch B it's 88 - 16 = 72."""
+    data = _audit_live()
+    files = {f["filename"]: f for f in data["files"]}
+    total = 0
+    for key in TARGETS:
+        findings = files[key]["findings"]
+        total += sum(1 for f in findings if f["category"] in MIG)
+    assert total == 72, f"expected total MIG=72 after Phase 4, got {total}"
+
+
+def test_phase4_modules_import_cleanly():
+    """Verify mcp_client module imports without errors after the 8 new _result variants."""
+    import src.mcp_client
+    assert hasattr(src.mcp_client, "get_git_diff_result")
+    assert hasattr(src.mcp_client, "ts_c_get_skeleton_result")
+    assert hasattr(src.mcp_client, "ts_c_get_code_outline_result")
+    assert hasattr(src.mcp_client, "ts_c_get_definition_result")
+    assert hasattr(src.mcp_client, "ts_c_get_signature_result")
+    assert hasattr(src.mcp_client, "ts_c_update_definition_result")
+    assert hasattr(src.mcp_client, "ts_cpp_get_skeleton_result")
+    assert hasattr(src.mcp_client, "ts_cpp_get_code_outline_result")