[client] pqkem: address review findings on convergence, strict parsing and status

Bot review (CodeRabbit, cubic) on PR #7098 surfaced several real defects:

- Strict() parsed the raw env value instead of the normalized one, so a
  mixed-case NB_PQ_MLKEM_STRICT such as "tRuE" silently disabled fail-closed
  mode. Parse the lower-cased value, matching Enabled().
- OnDataPathMessage labeled a data-path answer as "signal" in logs, mislabeling
  every rotation answer. Use the data-path label.
- A failed sendAnswer aborted the whole connection setup, skipping the relay/ICE
  listeners; a transient signalling failure now still brings the local transport
  up (the peer retries the answer).
- processOffer left the reserved exchange slot in place when Respond failed, so
  every retransmission of that offer was dropped forever. Clear the reservation
  on a pre-commit error so a retry can derive again.
- Dropped a no-op time.Since that implied a convergence-latency metric that was
  never recorded, and the now-unused startedAt field.
- The strict-mode status line asserted active blocking even for a peer that is
  simply offline; reword it to state that no PSK is established yet.
- conn_test used t.Fatalf for conditions under test; use assert.

client/status/status.go

@@ -793,7 +793,7 @@ func peerQuantumResistanceStatus(established, rosenpassEnabled, rosenpassPermiss
 	}
 	switch {
 	case mlkemEnabled && mlkemStrict:
-		return "false (ML-KEM strict: blocking peer traffic until the exchange converges)"
+		return "false (ML-KEM strict: no post-quantum PSK established for this peer yet)"
 	case mlkemEnabled:
 		return "false (ML-KEM: not converged yet, or peer does not run the exchange)"
 	case rosenpassEnabled && rosenpassPermissive: