[python] Add tests for Data Evolution BLOB writes on branches

[XiaoHongbo-Hope]

Purpose

Add regression coverage for writing BLOB columns to branches of Data Evolution tables.

Changes

• Add a filesystem catalog test that branches from a tag, writes a BLOB row, and verifies main/branch isolation.
• Route snapshot lookup and commit to the selected branch in the mock REST server.
• Add the equivalent mock REST catalog BLOB write and isolation test.

Only test code is changed.

Tests

• Python 3.11 / PyArrow 19: both new tests passed.
• Python 3.6 / PyArrow 6: both new tests passed.
• Flake8 and git diff --check passed.