Add single code paste validation

[jurei733]

Summary

• Add a Schemer-internal single-code copy buffer that survives iframe/component recreation within a browser session.
• Warn before pasting when copied code references target-sensitive structures such as arrays, fragments, numeric rules, or boolean rules.
• Re-run scheme validation after paste and show newly introduced validation problems.
• Cover copy/paste behavior, persistence, warning flow, and post-paste validation with unit tests.

Responses dependency

• This PR intentionally does not pin a concrete new @iqb/responses version yet.
• After the merged responses validation changes are published, update the dependency/lockfile in a follow-up commit or PR.

Validation

• npm run test:cc -- --watch=false --browsers=ChromeHeadless
• npm run lint (passes with one pre-existing max-len warning in generate-coding-dialog.component.spec.ts)
• npm run build_cc