fix(model): never invoke non-attribute methods on property access

[3m1n3nc3]

The model Proxy probed every accessed property by invoking the same-named method to check whether it returned an Attribute (a Laravel-style accessor). That fired the method's side effects on a bare property read:

• model.recordView ran recordView() before it was called, so model.recordView(viewer) (a read followed by a call) inserted twice — the reported 'relation-insert double-inserts' gotcha.
• Reading a relation like model.posts invoked posts() on every access.

resolveAttributeMutator now:

• only probes zero-arity methods (accessors take no args; actions/relations like recordView(viewer) never qualify),
• caches the accessor determination per class (an arity-0 method is probed at most once), and
• on the read path additionally requires the key to be a model attribute (loaded value, append, cast, or mapped column), so even a zero-arity side-effecting method is never fired by a property read. Writes still probe (the key being assigned is an attribute by definition), so Attribute-object set mutators keep working during hydration and on fresh models.

Tests: an argument-taking and a zero-arity side-effecting method are never invoked by property reads and run exactly once when called; Attribute-object get/set accessors and appends continue to resolve.

[codecov]

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✅ All modified and coverable lines are covered by tests.

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