Fix differential CI reference version drift

[ShawnChen-Sirius]

Fixes the scheduled differential CI failure caused by chdb-core 26.7.0 moving chDB's ClickHouse baseline to 26.7.2 while the workflow still extracted a fixed clickhouse-server:26.5.1.882 reference binary.

The differential job now installs the chDB test stack before choosing the reference binary, derives the ClickHouse Docker tag from SELECT version(), and falls back from X.Y.Z to X.Y when the exact patch tag is unavailable. The version-skew guard stays in place, but future chdb-core baseline bumps will pull a matching reference binary instead of failing on a stale hard-coded tag.

Verified with:

• pytest -q tests/differential
• ruff check tests/differential
• git diff --check
• confirmed clickhouse/clickhouse-server:26.7.2 exists on Docker Hub

Note

Fix differential CI version drift by detecting chDB's ClickHouse version at runtime

• The differential CI job previously used a hardcoded ClickHouse server tag, which could drift out of sync with the chDB build being tested.
• A new 'Detect chDB ClickHouse version' step runs SELECT version() via chdb and exports CHDB_CLICKHOUSE_VERSION, CLICKHOUSE_TAG_PATCH, and CLICKHOUSE_TAG_MINOR into the GitHub environment.
• The 'Extract clickhouse binary from server image' step now iterates over these derived tags instead of a fixed list, and emits a version-specific error if no matching tag is found.
• Test dependency installation (pip install -e .[dev,test-differential]) is moved earlier in the job, before the version detection step that requires chdb to be importable.

^{Macroscope summarized 5c4e96f.}