fix: sanitize calendar event headings and property values

`calendar-sync--event-to-org` already cleaned the description body via `calendar-sync--sanitize-org-body`, but the event summary went into the heading line and the location, organizer, status, and URL went into the property drawer without sanitization. Any of those fields containing newlines could create extra Org headings, close the property drawer early with a stray `:END:`, or inject property-looking lines that the agenda would then parse as real properties.

I added two helpers. `calendar-sync--sanitize-org-property-value` trims the input and collapses any run of whitespace or newlines into a single space. `calendar-sync--sanitize-org-heading` composes that over the existing body sanitizer so `*` sequences also become `-`. The event-to-org function now routes the summary through the heading sanitizer and each property value through the property sanitizer.

I added regression tests across two files. `test-calendar-sync--sanitize-org-body.el` gets 4 new tests for the two helpers, covering newline flattening, leading-star replacement, structural-character flattening, and whitespace collapse. `test-calendar-sync--event-to-org.el` gets 2 new integration tests. A summary containing `\n** Hidden task` produces a single `* ` heading with the body inlined. A location containing `\n:END:\n* Not a real heading` collapses to a single property line with no extra `:END:` or heading injected.

515 calendar-sync tests pass together.

1 files changed, 44 insertions, 0 deletions

diff --git a/tests/test-calendar-sync--event-to-org.el b/tests/test-calendar-sync--event-to-org.el
index a3dc0106..9f87856f 100644
--- a/tests/test-calendar-sync--event-to-org.el
+++ b/tests/test-calendar-sync--event-to-org.el
@@ -11,6 +11,16 @@
 (require 'testutil-calendar-sync)
 (require 'calendar-sync)
 
+(defun test-calendar-sync--count-line-matches (regexp text)
+  "Count lines in TEXT that match REGEXP."
+  (with-temp-buffer
+    (insert text)
+    (goto-char (point-min))
+    (let ((count 0))
+      (while (re-search-forward regexp nil t)
+        (setq count (1+ count)))
+      count)))
+
 ;;; Normal Cases
 
 (ert-deftest test-calendar-sync--event-to-org-normal-all-fields ()
@@ -108,6 +118,40 @@
       ;; Only the event heading should use *
       (should (= 1 (length (split-string result "^\\* " t)))))))
 
+(ert-deftest test-calendar-sync--event-to-org-boundary-summary-structure-is-flattened ()
+  "Test event summary cannot create additional Org headings."
+  (let* ((start (test-calendar-sync-time-days-from-now 5 14 0))
+         (end (test-calendar-sync-time-days-from-now 5 15 0))
+         (event (list :summary "* Planning\n** Hidden task"
+                      :start start
+                      :end end)))
+    (let ((result (calendar-sync--event-to-org event)))
+      (should (string-match-p "^\\* - Planning -- Hidden task$" result))
+      (should-not (string-match-p "^\\*\\* Hidden task" result))
+      (should (= 1 (length (split-string result "^\\* " t)))))))
+
+(ert-deftest test-calendar-sync--event-to-org-boundary-property-structure-is-flattened ()
+  "Test property values cannot create extra drawer lines or close the drawer."
+  (let* ((start (test-calendar-sync-time-days-from-now 5 14 0))
+         (end (test-calendar-sync-time-days-from-now 5 15 0))
+         (event (list :summary "Meeting"
+                      :start start
+                      :end end
+                      :location "Room 1\n:END:\n* Not a real heading"
+                      :organizer (list :cn "Jane\n:STATUS: fake"
+                                       :email "jane@example.com")
+                      :status "accepted\n:LOCATION: fake"
+                      :url "https://example.com/a\n:PROPERTIES:")))
+    (let ((result (calendar-sync--event-to-org event)))
+      (should (string-match-p ":LOCATION: Room 1 :END: \\* Not a real heading" result))
+      (should (string-match-p ":ORGANIZER: Jane :STATUS: fake" result))
+      (should (string-match-p ":STATUS: accepted :LOCATION: fake" result))
+      (should (string-match-p ":URL: https://example.com/a :PROPERTIES:" result))
+      (should (= 1 (test-calendar-sync--count-line-matches "^:LOCATION:" result)))
+      (should (= 1 (test-calendar-sync--count-line-matches "^:STATUS:" result)))
+      (should (= 1 (test-calendar-sync--count-line-matches "^:END:$" result)))
+      (should-not (string-match-p "^\\* Not a real heading" result)))))
+
 (ert-deftest test-calendar-sync--event-to-org-boundary-organizer-email-only ()
   "Test organizer without CN shows email."
   (let* ((start (test-calendar-sync-time-days-from-now 5 14 0))