[python] Avoid full-window conversion for first and last aggregates

[QuakeWang]

Purpose

Window first and last aggregates converted the entire window to Python objects before selecting one non-null value.

Scan Arrow chunks from the requested endpoint and convert only the selected value, preserving null and value semantics.

Tests

• Temporal suite: 76 passed, including chunk, null, nested-value, dictionary, and memory regressions.
• The memory regression fails on the previous implementation.
• Local Python allocation peak for 8,192 binary values of 1 KiB each: 8.38 MiB to about 1.6 KiB.
• Flake8, license headers, Python 3.6 syntax, and git diff --check passed.