fix: fail on SHA ref checkout instead of silently testing wrong commit

Shallow clones only fetch HEAD, so `git fetch --depth 1 origin <SHA>`
is needed for pinned refs. Previously, both fetch and checkout errors
were suppressed (2>/dev/null, || true), causing 4 extensions to
silently test at the wrong commit.

Now: check exit codes, use FETCH_HEAD for checkout, clean up cache on
failure to prevent stale state on --no-fetch reruns.

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

Author: ZviBaratz

scripts/field-test-runner.sh

@@ -155,8 +155,16 @@ resolve_ext_path() {
                     return 1
                 fi
                 if [[ -n "$ref" ]]; then
-                    git -C "$RESOLVED_PATH" fetch --depth 1 origin "$ref" 2>/dev/null
-                    git -C "$RESOLVED_PATH" checkout "$ref" 2>/dev/null || true
+                    if ! git -C "$RESOLVED_PATH" fetch --depth 1 origin "$ref" 2>&1; then
+                        echo "  SKIP: failed to fetch ref $ref from $repo"
+                        rm -rf "$RESOLVED_PATH"
+                        return 1
+                    fi
+                    if ! git -C "$RESOLVED_PATH" checkout FETCH_HEAD 2>&1; then
+                        echo "  SKIP: failed to checkout ref $ref"
+                        rm -rf "$RESOLVED_PATH"
+                        return 1
+                    fi
                 fi
             fi
             if [[ ! -d "$RESOLVED_PATH" ]]; then