Fix sp_LogHunter custom_message_only validation and canary date floor

Two related bugs in the setup path:

1. @custom_message_only = 1 with NULL/empty @custom_message produced an
   empty #search. The canary and canned-search inserts are gated on
   @custom_message_only = 0, so they skipped; the custom insert is
   gated on @custom_message LIKE N'_%' which is UNKNOWN (treated as
   FALSE) when @custom_message is NULL, so it also skipped. The outer
   cursor ran over nothing and the sproc returned "no results" on
   what should have been an invalid input. Added an early validation
   that RAISERRORs a clear message instead.

2. Canary searches built `days_back` as a "90 days ago at minimum"
   floor via CASE WHEN @days_back > -90 THEN -90 ELSE @days_back END.
   When the caller passes @start_date/@end_date instead of @days_back,
   the setup code NULLs @days_back — and CASE over NULL collapses to
   NULL, the CONVERT→DATEADD produces NULL, and the final concatenated
   literal becomes the string "NULL" (or NULLs all the way out), which
   xp_readerrorlog rejects. Switched to a CASE WHEN @days_back IS NOT
   NULL fallback that substitutes @start_date when date-range mode is
   active.

Verified on SQL Server 2022:
  @custom_message_only = 1 (no message) → expected error raised
  @start_date / @end_date set, @days_back NULL → canary rows build
  with a real date floor and xp_readerrorlog runs without error

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erikdarlingdata

sp_LogHunter/sp_LogHunter.sql

@@ -241,6 +241,21 @@ BEGIN
             @custom_message_only = 0;
     END;
 
+    /*
+    @custom_message_only = 1 means "skip the canned search strings and
+    look only for the user-supplied @custom_message". Without a message
+    to search for, every insert branch below would skip (the custom
+    insert is gated on @custom_message LIKE N'_%', which is NULL/false
+    for NULL or empty input), leaving #search empty and the whole
+    outer loop a no-op. Reject the combination up front.
+    */
+    IF @custom_message_only = 1
+    AND (@custom_message IS NULL OR LEN(@custom_message) = 0)
+    BEGIN
+        RAISERROR(N'@custom_message_only = 1 requires a non-empty @custom_message. Provide a search string or set @custom_message_only = 0.', 11, 1) WITH NOWAIT;
+        RETURN;
+    END;
+
     /*Fix @end_date*/
     IF  @start_date IS NOT NULL
     AND @end_date IS NULL
@@ -412,8 +427,24 @@ BEGIN
     CROSS JOIN
     (
         SELECT
+            /*
+            Canary floor is normally "at least 90 days back" so these
+            server-identity strings are found regardless of how recent
+            the caller is interested in. When the caller supplied
+            @start_date/@end_date, @days_back is NULL at this point —
+            the previous CASE collapsed to NULL, produced a NULL
+            days_back literal, and xp_readerrorlog received NULL as a
+            date argument and errored. Fall back to @start_date in
+            date-range mode so the canary has a concrete floor.
+            */
             days_back =
-                N'"' + CONVERT(nvarchar(10), DATEADD(DAY, CASE WHEN @days_back > -90 THEN -90 ELSE @days_back END, SYSDATETIME()), 112) + N'"',
+                N'"' +
+                CASE
+                    WHEN @days_back IS NOT NULL
+                    THEN CONVERT(nvarchar(10), DATEADD(DAY, CASE WHEN @days_back > -90 THEN -90 ELSE @days_back END, SYSDATETIME()), 112)
+                    ELSE CONVERT(nvarchar(10), @start_date, 112)
+                END +
+                N'"',
             start_date =
                 N'"' + CONVERT(nvarchar(30), @start_date) + N'"',
             end_date =