refactor(core): Centralize timestamp parsing in core::time

Duplicate ISO 8601 timestamp parsing functions existed in both
discussion.rs and merge_request.rs transformers. This extracts
iso_to_ms_strict() and iso_to_ms_opt_strict() into core::time
as the single source of truth, and updates both transformer
modules to use the shared implementations.

Also removes the private now_ms() from merge_request.rs in
favor of the existing core::time::now_ms(), and replaces the
local parse_timestamp_opt() in discussion.rs with the public
iso_to_ms() from core::time.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/time.rs

@@ -59,6 +59,22 @@ pub fn parse_since(input: &str) -> Option<i64> {
     iso_to_ms(input)
 }
 
+/// Convert ISO 8601 timestamp to milliseconds with strict error handling.
+/// Returns Err with a descriptive message if the timestamp is invalid.
+pub fn iso_to_ms_strict(iso_string: &str) -> Result<i64, String> {
+    DateTime::parse_from_rfc3339(iso_string)
+        .map(|dt| dt.timestamp_millis())
+        .map_err(|_| format!("Invalid timestamp: {}", iso_string))
+}
+
+/// Convert optional ISO 8601 timestamp to optional milliseconds (strict).
+pub fn iso_to_ms_opt_strict(iso_string: &Option<String>) -> Result<Option<i64>, String> {
+    match iso_string {
+        Some(s) => iso_to_ms_strict(s).map(Some),
+        None => Ok(None),
+    }
+}
+
 /// Format milliseconds epoch to human-readable full datetime.
 pub fn format_full_datetime(ms: i64) -> String {
     DateTime::from_timestamp_millis(ms)