fix(laravel): infer relation callbacks from argument types

[shuvroroy]

Relation callbacks lost their related model when the relation came from a variable, a union of names, or a relation object. Complete L60 by resolving the argument through the shared type engine and retaining each related model's custom builder. Apply the same inference to relation shortcuts and direct Builder::with() callbacks.

Reuse the existing argument binder, cached relation-chain resolver, and ancestor template projection. Custom relation subclasses can reorder or fix their generic parameters; eager callbacks preserve concrete relations, and morph callbacks retain their second parameter. Unknown eager names keep the declared callback type.

Adds 19 type assertions (113 total), 13 completion/diagnostic cases with positive and negative member checks, navigation tests, Laravel demos, and 13 runtime checks (217 total). The fixture includes Laravel's separate static Model::with($relations) declaration; the direct two-argument callback form is tested through query()->with() to match runtime behavior.

Stack: depends on #452 (L59), above #451, #450, #449, #448, and #447. This follow-up PR targets upstream main, so its diff includes the unmerged dependencies. The final commit contains L60; merge the dependencies first. L59 is unchanged.

Validation: full cargo test (14,837 passed), cargo clippy --fix --allow-dirty -- -D warnings, cargo clippy --all-targets -- -D warnings, cargo fmt --check, PHP syntax/runtime checks, and Laravel analysis with exactly the three intentional diagnostics.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 96.56160% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/type_engine/variable/closure_resolution.rs	94.91%	9 Missing ⚠️
...engine/variable/forward_walk/callable_inference.rs	98.80%	1 Missing ⚠️
src/virtual_members/laravel/builder.rs	98.38%	1 Missing ⚠️
src/virtual_members/laravel/relationships.rs	95.23%	1 Missing ⚠️

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