test: update 2026 TypeScript interop to beta.2

[leehack]

Summary

• Updates the 2026-07-28 RC TypeScript SDK interop fixture from @modelcontextprotocol/client / @modelcontextprotocol/server 2.0.0-beta.1 to the published 2.0.0-beta.2 packages.
• Refreshes the package lock plus interop docs, RC transition guide, fixture README, and changelog so the documented pin matches the executable fixture.

Upstream evidence

• npm view @modelcontextprotocol/client@2.0.0-beta.2 version exports --json confirms the beta.2 client package is published and still exposes the root import used by test/interop/ts_2026_07_28_rc/src/client.mjs.
• npm view @modelcontextprotocol/server@2.0.0-beta.2 version exports --json confirms the beta.2 server package is published and still exposes the root import used by test/interop/ts_2026_07_28_rc/src/server.mjs.
• Official TypeScript SDK release tags show @modelcontextprotocol/client@2.0.0-beta.2 and @modelcontextprotocol/server@2.0.0-beta.2 published on 2026-07-02.

Existing issue check

• Searched issues and PRs for 2.0.0-beta.2, @modelcontextprotocol/client beta.2, @modelcontextprotocol/server beta.2, TypeScript SDK beta.2, and ts_2026_07_28_rc.
• No dedicated open issue or PR already tracks the beta.2 fixture bump. Closest prior work is merged PR test: update 2026 interop and conformance pins #298, which moved the fixture to beta.1.

Test Plan

• cd test/interop/ts_2026_07_28_rc && npm ci
• package verification: installed @modelcontextprotocol/client and @modelcontextprotocol/server both report 2.0.0-beta.2
• dart test test/docs/markdown_docs_test.dart
• dart run tool/testing/run_ts_2026_07_28_rc_interop.dart
• dart analyze
• git diff --check origin/dev/2026-07-28-rc...HEAD