Implement timestamp_cast_dtype for SqliteDialect

[krinart]

Which issue does this PR close?

PR improves unparser to produce valid CAST SQL for SQLite and timestamp_text

Rationale for this change

SQLite does not have dedicated DATE type and prefers a text string that is an ISO 8601 date/time value.

Default implementation of timestamp_cast_dtype casts values to Timestamp. However you can see below that casting to TIMESTAMP produces invalid value in cast of sqlite, where casting to TEXT produces a valid one:

sqlite> select event_time, cast(event_time as timestamp), cast(event_time as text) from my_table limit 1;

2025-08-30 01:36:00.888580  |  2025  |  2025-08-30 01:36:00.888580

What changes are included in this PR?

PR introduces custom timestamp_cast_dtype implementation for SqliteDialect.

Are these changes tested?

Yes, added unit tests + manual testing

Are there any user-facing changes?

No

[Jefffrey]

Makes sense to me 👍