fix(pd): wait for KV transport readiness and track transfer progress

[WANDY666]

Wait for each KV transport module to finish initialization before consuming its results, and detect startup failures instead of waiting indefinitely. Pending decode pages use request progress for idle timeouts; active transfers retain their per-page deadline.

This prevents slow startup or long prefill from being mistaken for a failed transfer. Generation failures before the first streamed output return HTTP 500 instead of starting a successful response.

Tests: readiness, worker exit, bounded startup, progress timeout and HTTP error regressions (21 passed).