Inherit omit_if_default when generating dict unstructure hooks

[guhou-hvi]

Fixes #238.

With Converter(omit_if_default=True), manually registering a dict unstructure hook to rename a field currently makes default-valued fields reappear unless _cattrs_omit_if_default=True is repeated in the generator call.

make_dict_unstructure_fn and make_dict_unstructure_fn_from_attrs now inherit the converter's setting when this argument is omitted, following the existing "from_converter" convention. An explicit True or False still overrides the converter, and per-attribute overrides retain highest precedence. BaseConverter falls back to False. The setting is resolved when the hook is generated.

This also changes include_subclasses(..., overrides=...): its generated hooks now honor omit_if_default=True. If a default-valued Literal field is used to select a subclass during structuring, preserve it with a field override, for example overrides={"kind": override(omit_if_default=False)}. Other default-valued fields can still be omitted. The documentation and subclass round-trip tests cover this compatibility change.

Tests cover attrs classes and dataclasses, both generator entry points, default/true/false converter settings, explicit overrides, renamed fields, per-attribute precedence, and subclass round trips with default and tagged union strategies.
The 54 new cases produce 8 failures and 46 passes on the base commit.

Validation on Windows/Python 3.12.2:

• Core suite: 984 passed, 15 expected failures (tests, excluding tests/test_preconf.py and tests/preconf).
• Ruff 0.15.6, as pinned in uv.lock: full project lint and format checks pass.
• The original issue reproduction now passes; generated hook bytecode, constants and defaults match the baseline for unchanged default/explicit-boolean behavior.
• Sphinx: all 78 doctests in the changed customizing page pass and the new API links resolve. Full HTML/doctest runs reproduce the baseline's seven missing optional-module warnings and four doctest failures (two Windows path representations, two from missing orjson).

The optional preconfigured-converter suites and other Python versions were not run locally.

AI assistance: Codex drafted the implementation, tests, documentation and this description, and ran the local checks. A separate Codex agent reviewed the change; its compatibility finding was addressed. I reviewed and understand the changes.

[codspeed]

Merging this PR will not alter performance

✅ 64 untouched benchmarks

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_{Comparing guhou-hvi:fix/omit-if-default-238 (ff4682a) with main (b3fb197)}

[Tinche]

LGTM! Thanks.