Honor a default_cache_ttl_seconds of 0 in Prompts

[ckarnell]

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Prompts.__init__ resolves the default cache TTL with default_cache_ttl_seconds or DEFAULT_CACHE_TTL_SECONDS, so passing 0 is treated as if nothing was passed and the value becomes 300.

0 is a meaningful setting here, and the same class already honors it on the other path. _get_internal resolves the per-call TTL with cache_ttl_seconds if cache_ttl_seconds is not None else ..., and is_fresh = (now - fetched_at) < ttl with ttl=0 means "never serve from cache". So the per-call option lets you disable caching with 0 while the constructor default silently cannot.

Measured against the library, with no network: Prompts(default_cache_ttl_seconds=0) reports a TTL of 300. Counting fetch attempts against a seeded cache, the constructor 0 serves from cache (0 fetches) identically to passing no TTL at all, while the per-call get(cache_ttl_seconds=0) correctly refetches.

The fix matches the per-call form, +3/-1:

self._default_cache_ttl_seconds = (
    default_cache_ttl_seconds
    if default_cache_ttl_seconds is not None
    else DEFAULT_CACHE_TTL_SECONDS
)

I added a regression test that fails on the current code (AssertionError: 1 != 2) and passes with the fix. The existing suite is 68 passing before and 69 after, so this path was never covered. They test a TTL of 60. Never 0. None still yields 300 and default behavior does not change.

I did not run against a live PostHog instance, so all measurements use the repo's mocked session or a seeded cache.