fix(metrics): validate time conversion results before API calls

Add explicit || exit 1 and empty-string checks after range_to_rfc3339
and date calls. Prevents empty timestamps from reaching the API with
confusing errors when time conversion fails silently.

Thread: https://ampcode.com/threads/T-019c5138-6cb8-7769-a841-0cba5cb26fd1

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019c5138-6cb8-7769-a841-0cba5cb26fd1

Author: Licenser

scripts/axiom-metrics-discover

@@ -84,17 +84,25 @@ if [[ -n "$RANGE" && ( -n "$START_TIME" || -n "$END_TIME" ) ]]; then
 fi
 
 if [[ -n "$RANGE" ]]; then
-  START_TIME=$(range_to_rfc3339 "$RANGE")
-  END_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ)
+  START_TIME=$(range_to_rfc3339 "$RANGE") || exit 1
+  END_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ) || exit 1
+  if [[ -z "$START_TIME" || -z "$END_TIME" ]]; then
+    echo "Error: Failed to compute time range from '$RANGE'." >&2
+    exit 1
+  fi
 elif [[ -n "$START_TIME" && -n "$END_TIME" ]]; then
   : # explicit start/end provided
 elif [[ -n "$START_TIME" || -n "$END_TIME" ]]; then
   echo "Error: Both --start and --end are required when specifying explicit times." >&2
   exit 1
 else
   # Default to 1h
-  START_TIME=$(range_to_rfc3339 "1h")
-  END_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ)
+  START_TIME=$(range_to_rfc3339 "1h") || exit 1
+  END_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ) || exit 1
+  if [[ -z "$START_TIME" || -z "$END_TIME" ]]; then
+    echo "Error: Failed to compute default time range." >&2
+    exit 1
+  fi
 fi
 
 # URL-encode a path segment

scripts/axiom-metrics-query

@@ -116,8 +116,12 @@ if [[ -n "$RANGE" && ( -n "$START_TIME" || -n "$END_TIME" ) ]]; then
 fi
 
 if [[ -n "$RANGE" ]]; then
-  START_TIME=$(range_to_rfc3339 "$RANGE")
-  END_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ)
+  START_TIME=$(range_to_rfc3339 "$RANGE") || exit 1
+  END_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ) || exit 1
+  if [[ -z "$START_TIME" || -z "$END_TIME" ]]; then
+    echo "Error: Failed to compute time range from '$RANGE'." >&2
+    exit 1
+  fi
 elif [[ -z "$START_TIME" || -z "$END_TIME" ]]; then
   echo "Error: Either (--start + --end) or --range is required." >&2
   exit 1