org-agenda: Match timestamps inside node properties

* lisp/org-agenda.el (org-agenda-get-timestamps): Use
`org-at-timestamp-p' instead of org-element to check if at valid
timestamp.  Agenda should consider timestamps inside node properties
even though they are not parsed by org-element.

* testing/lisp/test-org-agenda.el (test-org-agenda/property-timestamp):
Add new test checking active timestamp inside property drawer.

* testing/examples/agenda-file.org (test timestamp inside properties):
Add heading for `test-org-agenda/property-timestamp'.

Reported in https://list.orgmode.org/06d301d83d9e$f8b44340$ea1cc9c0$@tomdavey.com

lisp/org-agenda.el

@@ -5732,7 +5732,7 @@ displayed in agenda view."
 		    (org-before-first-heading-p)
 		    (and org-agenda-include-inactive-timestamps
 			 (org-at-clock-log-p))
-                    (not (eq 'timestamp (org-element-type (org-element-context)))))
+                    (not (org-at-timestamp-p 'agenda)))
 	    (throw :skip nil))
 	  (org-agenda-skip))
 	(let* ((pos (match-beginning 0))

testing/examples/agenda-file.org

@@ -16,3 +16,7 @@ SCHEDULED: <2022-01-03 Mon>
 #+begin_example
 SCHEDULED: <2022-01-03 Mon>
 #+end_example
+* test timestamp inside properties
+:PROPERTIES:
+:CREATED: <2022-03-22 Tue>
+:END:

testing/lisp/test-org-agenda.el

@@ -106,6 +106,21 @@ See https://list.orgmode.org/20220101200103.GB29829@itccanarias.org/T/#t."
     (should (= 2 (count-lines (point-min) (point-max)))))
   (org-test-agenda--kill-all-agendas))
 
+(ert-deftest test-org-agenda/property-timestamp ()
+  "Match timestamps inside property drawer.
+See https://list.orgmode.org/06d301d83d9e$f8b44340$ea1cc9c0$@tomdavey.com"
+  (cl-assert (not org-agenda-sticky) nil "precondition violation")
+  (cl-assert (not (org-test-agenda--agenda-buffers))
+	     nil "precondition violation")
+  (let ((org-agenda-span 'day)
+	(org-agenda-files `(,(expand-file-name "examples/agenda-file.org"
+					       org-test-dir))))
+    (org-agenda-list nil "<2022-03-22 Tue>")
+    (set-buffer org-agenda-buffer-name)
+    (message "%s" (buffer-string))
+    (should (= 4 (count-lines (point-min) (point-max)))))
+  (org-test-agenda--kill-all-agendas))
+
 (ert-deftest test-org-agenda/set-priority ()
   "One informative line in the agenda. Check that org-agenda-priority updates the agenda."
   (cl-assert (not org-agenda-sticky) nil "precondition violation")