TimestampParser does not support full ISO8601

**Describe the bug**

The documentation claims to support "RFC3339 / ISO8601 standard timestamps", but it will not accept strings with missing time components, for example `hh::mm` format (missing seconds).

Can the documentation be clarified, or the full format list supported?

**To Reproduce**

In `datafusion-cli`:

```
> SELECT to_timestamp_micros('2025-08-14T00:00Z') as timestamp_col;
Arrow error: Parser error: Error parsing timestamp from '2000-01-01T00:00Z': error parsing time
```

**Expected behavior**

**Additional context**

Start by reproducing the `to_timestamp_micros` query in `datafusion-cli` and trace the `TimestampParser` handling for `2025-08-14T00:00Z`. Review the timestamp documentation and parsing entry point to determine the supported ISO8601 forms; done means either the documented format list matches actual behavior or the reported missing form is accepted with coverage for the case.

Written by the indexing model from the issue text.