fix: preserve context immutability in parse functions

[bgk614]

Summary

Fixes mutation of user-provided ctx parameter in parse functions by replacing Object.assign with object spread syntax.

Problem

Using Object.assign(_ctx, {…}) mutates the original context object passed by the user.

Before Example:

const ctx: schemas.ParseContextInternal = _ctx ? Object.assign(_ctx, { async: false }) : { async: false };

After Example:

const ctx: schemas.ParseContextInternal = _ctx ? { ..._ctx, async: false } : { async: false };

Affected Functions

• _parse
• _parseAsync
• _safeParseAsync
• _encode
• _encodeAsync
• _safeEncode
• _safeEncodeAsync

Note: _safeParse already used spread operator.

Changes

• packages/zod/src/v4/core/parse.ts - Replace Object.assign with spread operator
• packages/zod/src/v4/classic/tests/codec.test.ts - Add context immutability tests

Fixes #5466

[pullfrog]

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