test(config-utilities): cover validate-timestamps and format-report helpers

Two test files covering the extracted timestamp-validation helpers.

cj/--validate-timestamps-in-buffer (8 tests): empty buffer no-op,
buffer with no timestamps, buffer with all valid timestamps,
DEADLINE flagged with "DEADLINE" property, SCHEDULED flagged with
"SCHEDULED", inline-timestamp flagged with "inline timestamp",
multiple invalid collected in document order, mixed valid+invalid
returning only the invalid one. Tests use real org parsing and
mock org-time-string-to-absolute at the boundary so an arbitrary
timestamp can be marked invalid for a given test.

cj/--format-validation-report-section (4 tests): no-entries says
"No invalid timestamps found", single-entry produces the file: link
+ Property/Type + Invalid timestamp lines, multiple-entry preserves
input order, every section ends with a trailing blank line.

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