Fix macOS gateway exec approvals to respect exec-approvals.json.
This updates the macOS gateway prompter to resolve per-agent exec approval policy before deciding whether to show UI, use agentId for policy lookup, honor askFallback when prompts cannot be presented, and resolve no-prompt decisions from the configured security policy instead of hardcoded allow-once behavior. It also adds regression coverage for ask-policy and allowlist-fallback behavior, plus a changelog entry for the fix.
Co-authored-by: ImLukeF <92253590+ImLukeF@users.noreply.github.com>