fix(server): accept structurally compatible Zod v4 schemas

[Genmin]

Summary

Fixes #1987 on the v1.x branch by avoiding a type-identity leak from the SDK's own zod/v4/core import in the public Zod compatibility types.

AnySchema now accepts Zod v4 schemas structurally via the _zod.input / _zod.output contract. This preserves inference for registerTool raw shapes while allowing projects whose package manager resolves a different compatible Zod v4 copy than the SDK's copy.

Validation

• Reproduced the issue in an external consumer with top-level zod@4.4.1 and a nested SDK-resolved zod@4.3.6: registerTool failed with ZodString not assignable to AnySchema.
• Packed this branch and re-ran the same duplicate-Zod consumer repro: npx tsc --noEmit passed and callback args inferred correctly.
• npm run typecheck
• npm run build
• npm test -- test/issues/test_1987_zod_v4_type_identity.test.ts test/server/mcp.test.ts test/server/completable.test.ts
• npm run lint

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🦋 Changeset detected

Latest commit: bfc7eb8

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commit: bfc7eb8

[Genmin]

Added the missing patch changeset for the v1.x SDK package so the release-metadata blocker should clear on the next checks run. The code checks were already green before this metadata-only update.