Translate three-argument ConvertTimeBySystemTimeZoneId

[roji]

Allow queries to convert a local timestamp using an explicit source timezone, including timezone IDs stored in columns, via TimeZoneInfo.ConvertTimeBySystemTimeZoneId(dateTime, sourceTimeZoneId, destinationTimeZoneId).

Require the input to be mapped to timestamp without time zone and translate to (value AT TIME ZONE source) AT TIME ZONE destination. Both timezone arguments can be constants, parameters, or columns. The result always maps to timestamp without time zone, even for UTC destinations, consistent with the existing two-argument translation. Callers needing a UTC DateTime/timestamptz result can compose with the existing DateTime.SpecifyKind(..., DateTimeKind.Utc) translation.

Keep the DateTimeOffset overload unsupported because its nonzero-offset result cannot be represented by PostgreSQL. Includes coverage for UTC and non-UTC destinations, parameter and column timezones, UTC result composition, and unsupported inputs.

Fixes #3938

[Copilot]

🟢 Approval recommended

The implementation matches the requested semantics and includes comprehensive coverage.

0 open findings

What changed in this PR

Adds translation for three-argument TimeZoneInfo.ConvertTimeBySystemTimeZoneId using nested PostgreSQL AT TIME ZONE operations.

Changes:

• Supports constant, parameter, and column timezone IDs.
• Enforces timestamp without time zone inputs and preserves timestamp result mapping.
• Adds coverage for conversions, UTC composition, and unsupported inputs.

File	Description
NpgsqlDateTimeMethodTranslator.cs	Implements and validates the new translation.
TimestampTranslationsTest.cs	Adds functional and failure-case coverage.

🧠 Review effort: Balanced

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