### What changes were proposed in this pull request?
In the PR, I propose to move interval parsing to `CalendarInterval.fromCaseInsensitiveString()` which throws an `IllegalArgumentException` for invalid strings, and reuse it from `CalendarInterval.fromString()`. The former one handles `IllegalArgumentException` only and returns `NULL` for invalid interval strings. This will allow to support interval strings without the `interval` prefix in casting strings to intervals and in interval type constructor because they use `fromString()` for parsing string intervals.
For example:
```sql
spark-sql> select cast('1 year 10 days' as interval);
interval 1 years 1 weeks 3 days
spark-sql> SELECT INTERVAL '1 YEAR 10 DAYS';
interval 1 years 1 weeks 3 days
```
### Why are the changes needed?
To maintain feature parity with PostgreSQL which supports interval strings without prefix:
```sql
# select interval '2 months 1 microsecond';
interval
------------------------
2 mons 00:00:00.000001
```
and to improve Spark SQL UX.
### Does this PR introduce any user-facing change?
Yes, previously parsing of interval strings without `interval` gives `NULL`:
```sql
spark-sql> select interval '2 months 1 microsecond';
NULL
```
After:
```sql
spark-sql> select interval '2 months 1 microsecond';
interval 2 months 1 microseconds
```
### How was this patch tested?
- Added new tests to `CalendarIntervalSuite.java`
- A test for casting strings to intervals in `CastSuite`
- Test for interval type constructor from strings in `ExpressionParserSuite`
Closes#26079 from MaxGekk/interval-str-without-prefix.
Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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