Switch to pnpm and fix dependency security vulnerabilities

[dillonstreator]

Summary

• Migrate from Yarn classic to pnpm and clear known dependency security vulnerabilities (Express, OpenTelemetry, Babel, fetch-mock, and related transitive overrides).
• Update OpenTelemetry usage for the current SDK APIs and bump fetch-mock tests for v12.
• Add Changesets for versioning/changelogs/GitHub Releases (no npm publish) plus CI build/test and release workflows.

Test plan

• pnpm install
• pnpm test
• pnpm build
• pnpm audit reports no known vulnerabilities
• Confirm CI workflow passes on this PR
• After merge, confirm Release workflow opens a Version Packages PR from the included changeset

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