Refactor serdes branch coverage tests into native pytest cases

Replace the manual _unittest_serdes_branch_coverage_tests dispatcher with directly collected _unittest_* tests so pytest executes each case natively. This removes hand-maintained invocation logic and makes failures easier to localize at the test-case level.

Use pytest parameterization for the previously manually looped scenarios (float special values, bool conversion edge cases, width/cast-mode matrices, byte container variants, API str/bytes permutations, and aligned bit roundtrips).

Add focused bit-level tests that cover the remaining _BitWriter overwrite branches and _BitReader unaligned out-of-bounds zero-extension branch, restoring 100% branch coverage for _serdes.py while keeping strict mypy compatibility via a typed parameterization helper.

Author: pavel-kirienko