Refactor: extract pure-logic base classes to prepare for async implementation (#167)

## Summary

Extracts pure logic (URL building, payload construction, parsing,
caches, multipart serialization) from the sync clients into shared base
classes `_ODataBase` and `_BatchBase`. The sync clients now inherit from
these bases. The async client (Phase 2) will inherit from the same bases
as a sibling, enabling code sharing without the async client being a
subtype of the sync client.

- `_ODataBase` — pure logic extracted from `_ODataClient`; owns cache
teardown and HTTP logger lifecycle via `close()`
- `_BatchBase` — pure logic extracted from `_BatchClient`; owns
multipart serialization and response parsing

## Tests
1369 passed
Total coverage: 94.35% (threshold: 90%)

Co-authored-by: Abel Milash <abelmilash@microsoft.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people