fix: PostgreSQL dialect can not support tinyint type

[xiedeyantu]

Which issue does this PR close?

• No linked issue.

Rationale for this change

DataFusion's PostgreSQL unparser should emit PostgreSQL-compatible SQL for integer casts.Int8 was still being rendered as TINYINT, which is not valid PostgreSQL syntax. This change makes PostgreSQL output SMALLINT instead.

What changes are included in this PR?

• Added an int8_cast_dtype hook to the SQL unparser dialect abstraction.
• Updated PostgreSqlDialect to map Int8 to SMALLINT.
• Routed DataType::Int8 unparsing through the dialect hook.
• Added a regression test covering select cast(3 as tinyint)] with PostgreSQL unparser output.

Are these changes tested?

• Yes. Ran:
  • cargo test -p datafusion-sql --test sql_integration cases::plan_to_sql::test_cast_to_tinyint -- --exact
• The test passes.

Are there any user-facing changes?

• Yes. PostgreSQL dialect SQL generation now renders CAST(... AS SMALLINT) for Int8 values instead of CAST(... AS TINYINT).