Factory count-conditional return types

[shuvroroy]

create()/make() on an Eloquent factory now resolve to the model or to a collection of models depending on whether the call-site chain set a count. See the commit message for the reasoning and the edge cases.

Worth a reviewer's attention:

• The count state is read syntactically off the receiver chain, so a factory that travels through a variable keeps the single-model reading. That is the deliberate limit of the approach, not an oversight.
• factory($count) only counts for a literal numeric. Larastan resolves the argument's type; we don't, and guessing wrong on a variable would silently turn one model into a collection.
• The Factory test stub in tests/integration/completion_laravel.rs gained Laravel's real signatures (?int $count, times(), new(), the Collection<int, TModel>|TModel union). Existing factory tests pass against it unchanged.
• test_factory_convention_based_chain_count_then_create asserted the old behaviour (count(3)->create() → User) and was replaced.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json) — n/a, no new config
• I have added/updated tests to cover my changes

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