String-to-timestamp cast diverges from Spark's parseTimestampString: rejects 1-digit segments and empty fractions, accepts zone suffixes on date-only strings and 7-digit years

[peterxcli]

Describe the bug

Comet's string→timestamp parser is a fixed list of anchored regexes (native/spark-expr/src/conversion_funcs/string.rs:1646-1660) plus an offset-suffix fallback (:1471-1475, :1567-1635). Spark's SparkDateTimeUtils.parseTimestampString is a segment scanner with per-segment digit rules (isValidDigits). They disagree in four ways, all on the default-on path, for both TIMESTAMP and TIMESTAMP_NTZ (LEGACY returns NULL, ANSI raises CAST_INVALID_INPUT):

1. Month/day/hour/minute/second must be exactly 2 digits in Comet; Spark accepts 1–2 digits: '2020-1-1', '2020-01-01 12:34:5' → Spark valid, Comet NULL / error.
2. Empty fraction: '2020-01-01 12:34:56.' → Spark valid, Comet NULL (RE_MICROSECOND requires \.\d+).
3. Zone suffix on a date-only string: '2020-10-01Z', '2020-01-01+05:30', '2020-10-01 UTC' → Spark NULL (a zone is only allowed after a time segment), Comet accepts them (midnight in that zone).
4. 7-digit years: Spark's timestamp maxDigitsYear is 6 (only stringToDate allows 7); '0002020-01-01 00:00:00' → Spark NULL, Comet 2020-01-01 00:00:00.

Steps to reproduce

CREATE TABLE t USING parquet AS SELECT * FROM VALUES
  ('2020-1-1'), ('2020-01-01 12:34:5'), ('2020-01-01 12:34:56.'), ('2020-10-01Z'), ('0002020-01-01 00:00:00') AS t(s);
SELECT s, CAST(s AS TIMESTAMP) FROM t;

input	Spark (UTC)	Comet
2020-1-1	2020-01-01 00:00:00	NULL
2020-01-01 12:34:5	2020-01-01 12:34:05	NULL
2020-01-01 12:34:56.	2020-01-01 12:34:56	NULL
2020-10-01Z	NULL	2020-10-01 00:00:00
0002020-01-01 00:00:00	NULL	2020-01-01 00:00:00

With spark.sql.ansi.enabled=true the first three fail the whole query in Comet.

Expected behavior

The same accept/reject set as Spark.

Proposed solution

Port Spark's byte-scanning segment machine (segments 0–8 with per-segment digit-count validation; a zone suffix only after a time segment) for both the TZ and NTZ parsers. At minimum: relax month/day/hour/minute/second to \d{1,2}, allow an empty fraction, cap timestamp years at 6 digits, and only attempt suffix extraction when the remainder ends after a time segment. Add these strings to the cast suite — the fuzz alphabet 0123456789/:T contains neither - nor ., so it cannot generate them.

Additional context

Open PR #5130 re-implements the same 14 shapes as a byte classifier with bit-identical output, so it preserves these divergences. #5165 tracks sibling trimming divergences in the same parser; #3776 asks to port DateTimeUtilsSuite, which would surface some of these.