encoding/json/v2: optimize IsZero method resolution for omitzero (fix #80868)

[SparshGarg999]

Fixes #80868

Context & Problem

When marshalling structs tagged with omitzero using encoding/json/v2, if a field's underlying type implements IsZero() bool on a pointer receiver (e.g., func (t *MyType) IsZero() bool), encoding/json/v2 previously evaluated value IsZero method resolution before checking pointer receiver methods. This caused reflection to construct an addressable copy on the heap, triggering two extra allocations per field during struct marshalling.

Solution & Changes

• Reordered IsZero method resolution in src/encoding/json/v2/fields.go to test for pointer-receiver IsZero implementations first before value-receiver fallback.
• Eliminates temporary heap allocations when evaluating omitzero fields implementing IsZero.

Testing & Verification

• Benchmarked json.Marshal allocations on omitzero structs with pointer-receiver IsZero methods, verifying zero extra allocations.

[gopherbot]

This PR (HEAD: 27cb769) has been imported to Gerrit for code review.

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