test(web): add unit tests for terminal paste guard

[dylantirandaz]

What this changes

Adds unit tests for apps/web/src/features/terminal/paste-guard.ts, the paste-safety heuristics module that decides whether clipboard text needs an explicit confirmation before reaching a PTY. The module had no test coverage; this PR adds tests only — no source behavior changes.

The colocated paste-guard.test.ts covers the observable contract:

• Each of the 11 destructive patterns matches representative input and surfaces its exact deduplicated label (e.g. rm -rf /tmp/x → "force-deletes files", curl example.com/x | sh → "pipes a download into a shell", :(){ :|:& };: → "is a fork bomb"), including label ordering and deduplication when a pattern matches twice.
• Negative cases: benign single-line input (ls -la) requires no confirmation, and ordinary prose / safe commands that the regexes intentionally don't match (e.g. "the rm command is dangerous", "read the sudoers file", "rebooting the conversation", curl example.com without a pipe) stay clean.
• Boundaries: exactly LARGE_PASTE_CHARS chars is large, one char under is not; a single trailing newline counts as multiline; the empty string reports 0 lines and no confirmation.
• preparePasteForPty: CRLF, LF, and mixed newlines normalize to CR; lone CRs pass through.
• pastePreview: input over PASTE_PREVIEW_CHARS truncates with truncated: true, input over 6 lines truncates to 6 lines, and short input passes through with truncated: false.

Verification

• pnpm --filter @t4-code/web test — 72 test files, 1038 tests, all passed (new file included; 43 tests in paste-guard.test.ts alone).
• pnpm check — passed (release contract, provenance, lint, typecheck).

Checklist

• pnpm check passes (release contract, provenance, lint, typecheck)
• pnpm test passes; new behavior has a test that fails without this change
• Any pasted logs or screenshots are redacted: no secrets, tokens, pairing codes, or private paths
• Ported code (T3 Code / OMP) follows the provenance rules in THIRD_PARTY_NOTICES.md