Prompt-cache-break diagnostics reworked, with a new bypass path for one source

Claude Code's internal logic for detecting "prompt cache breaks" (when a cached prompt prefix stops being reusable) was reorganized: the threshold logic for deciding when a break occurred now lives in a shared helper. A new branch handles the case where a specific message source reports no session state, logging a diagnostic event with request ID and cache token counts instead of just returning early. There's also a new early exit that skips break diagnosis entirely when the cache state was seeded from disk.