SQLAlchemy compliance: support datetime literals and timestamp microseconds

[laughingman7743]

Context

Parent issue: #736

The dialect closes datetime_literals and timestamp_microseconds.
It also deletes DateTimeMicrosecondsTest and TimestampMicrosecondsTest, although Athena engine v3 supports timestamp literals and sub-second precision.

Scope

• Verify DATE and TIMESTAMP literal rendering against Athena engine v3.
• Verify reflected and round-tripped timestamp precision.
• Open only the requirements supported by Athena.
• Re-enable the relevant compliance classes and gate unsupported cases narrowly.

Acceptance criteria

• Supported datetime literal and timestamp microsecond tests pass.
• Compiler tests cover DATE and TIMESTAMP literal SQL without AWS where practical.
• Any Athena precision limit is documented in the requirement gate or test skip reason.
• just lint passes.
• Athena tests are limited to the affected nodes and run with -n 1.