Fix slash escaping in entity keys and navigation paths

[Shubham-Padkonde]

An entity key such as /BAZ/FOO currently leaves its slashes unescaped, which can make the server interpret part of the key as a path. Encode slashes inside OData quoted literals while retaining separators outside them. This also handles doubled apostrophes and preserves the encode_path=False opt-out.

Fixes #282. The earlier attempt in #284 identified navigation as a limitation; the literal-aware encoding here covers both collection and single-entity navigation without encoding their separators. Tests also cover changeset/batch serialization and decoding, percent signs, and GET/update/delete paths.

Validation: all 312 tests pass (pytest --cov-report term --cov=pyodata), Pylint scores 10/10, pinned Flake8 passes, and git diff --check passes. Seven initial regression cases failed before the fix.

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[phanak-sap]

The PR is fixing only outbound part, ignoring necessary changes for inbound direction.

Check #309 and #313.