chore: prepare 0.7.1 release

[JJasonSun]

Summary

• Set the package version to 0.7.1.
• Move the current Unreleased entries into the 0.7.1 changelog section.

This PR only prepares the release. Merging it does not publish to npm.

Checklist

• I kept this change within the documented server SDK scope.
• I did not add browser/client-side patterns that expose CALL-E API keys.
• I did not include private collaboration links or unconfirmed public repository references.
• I updated tests, examples, or docs when behavior changed.
• I updated the changelog when the change affects package users.
• I ran the relevant local checks.

Local checks

npx --yes pnpm@8.10.2 install --frozen-lockfile
npx --yes pnpm@8.10.2 run validate
npx --yes pnpm@8.10.2 audit --prod --audit-level high
node scripts/require-newer-stable-version.mjs 0.7.1 0.7.0

@call-e/calle@0.7.1 is currently unpublished on npm.