test(core): enforce transport exact optional compatibility

[rechedev9]

Summary

• add a dedicated exactOptionalPropertyTypes compile check for the Transport compatibility case
• wire that check into @modelcontextprotocol/core's existing typecheck script
• fix the adjacent createFetchWithInit RequestInit.headers assignment that fails once exact optional property types are actually enforced

Why

PR #1766 fixes the public type surface, but without compiling a targeted check under exactOptionalPropertyTypes: true the regression can still slip back in. When that check is enabled, createFetchWithInit also needs a small adjustment because assigning headers: undefined into RequestInit is rejected under exact optional property semantics.

Verification

• pnpm --filter @modelcontextprotocol/core typecheck
• pnpm --filter @modelcontextprotocol/core test -- transport.types.test.ts
• pnpm --filter @modelcontextprotocol/core lint

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@modelcontextprotocol/core	Patch

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