Fix PostgreSQL proxy timestamp text output adding trailing .0

[daguimu]

Problem

When querying a TIMESTAMP column without fractional seconds through ShardingSphere-Proxy (PostgreSQL protocol), the result includes a trailing .0 (e.g., 1973-06-03 10:30:01.0), whereas native PostgreSQL returns 1973-06-03 10:30:01. This formatting mismatch can break clients and tests relying on stable textual output.

Root Cause

In PostgreSQLDataRowPacket#writeTextValue, non-binary column values fall through to the generic each.toString() branch. When the value is a java.sql.Timestamp, Timestamp.toString() always emits a fractional part (at minimum .0), which does not match PostgreSQL's text protocol behavior of omitting trailing zero fractional seconds.

Fix

• Added a type-aware branch for java.sql.Timestamp in writeTextValue that formats the timestamp according to PostgreSQL text output rules:
  • Omits the fractional part entirely when there are no fractional seconds
  • Uses microsecond precision (matching PostgreSQL's internal precision) and strips trailing zeros from the fractional part
• The formatting logic is encapsulated in a private formatTimestamp method within PostgreSQLDataRowPacket

Tests Added

Change Point	Test
Timestamp without fractional seconds omits .0	timestamp_no_fractional — verifies 1973-06-03 10:30:01 (exact reproduction from issue)
Timestamp with fractional seconds preserved	timestamp_with_fractional — verifies 2024-01-15 14:30:45.123
Trailing zeros in fractional part stripped	timestamp_trailing_zeros_stripped — verifies .120 becomes .12
Full microsecond precision supported	timestamp_microsecond_precision — verifies 2024-01-15 14:30:45.123456

Impact

Only affects PostgreSQL text protocol timestamp formatting in the proxy. No changes to binary protocol, other data types, or any other module. Existing behavior for all non-Timestamp types is unchanged.

Fixes #37437