fix: avoid DOM-only HeadersInit in v1 declarations

[kocaemre]

Summary

Fixes the v1 declaration output so Node-only TypeScript projects do not need the DOM lib just to consume normalizeHeaders.

The generated shared/transport.d.ts currently exposes the bare global HeadersInit, which is not provided by @types/node even when Node fetch globals are enabled. This changes the public signature to RequestInit['headers'], matching the same runtime inputs without pulling in a DOM-only alias.

Also updates two internal client header annotations for the same reason and adds a patch changeset.

Verification

npm run build:esm
npm run build:cjs
npm run typecheck
npm run lint
npm test -- test/client/sse.test.ts test/client/streamableHttp.test.ts

Node-only declaration smoke test:

# temp project with:
#   lib: ["ES2023"]
#   types: ["node"]
#   skipLibCheck: false
#   @types/node@22.20.1
#   TypeScript 5.9

import { normalizeHeaders } from "@modelcontextprotocol/sdk/shared/transport";

const headers = normalizeHeaders({ "x-test": "value" });
headers["x-test"].toUpperCase();

Result: npx tsc -p tsconfig.json passes.

Fixes #2568

[changeset-bot]

🦋 Changeset detected

Latest commit: 6b74bc7

The changes in this PR will be included in the next version bump.

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