fix(pairing): retry get_key_tokens past a transient empty result

pair_spire failed with "bunker returned no tokens after create_new_token" when
the nsecbunkerd was briefly slow — get_key_tokens listed nothing in the few ms
after create_new_token before the write landed. Observed live: a nip44_decrypt
timeout, then a 502 on pair, then the identical call 25s later succeeding.

Retry get_key_tokens up to 5x with a 0.4s backoff before giving up, so pairing
survives a sluggish bunker instead of flaking. Tests cover the transient-empty
recovery and the exhausted-attempts failure.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_pairing.py

@@ -248,6 +248,50 @@ def test_malformed_token_raises():
         _pair(bunker)
 
 
+def test_get_key_tokens_retries_past_a_transient_empty(monkeypatch):
+    # A slow bunker can list no tokens right after create_new_token; retry.
+    import spirekeeper.pairing as pairing_mod
+
+    async def _no_sleep(*_a, **_k):
+        pass
+
+    monkeypatch.setattr(pairing_mod.asyncio, "sleep", _no_sleep)
+
+    bunker = FakeBunker(token_secret="s")  # pragma: allowlist secret
+    real = bunker.get_key_tokens
+    calls = {"n": 0}
+
+    async def flaky(key_name):
+        calls["n"] += 1
+        return [] if calls["n"] == 1 else await real(key_name)
+
+    bunker.get_key_tokens = flaky
+    result = _pair(bunker)
+    assert calls["n"] == 2  # empty once, then the token
+    assert result.spire_npub == _SPIRE_NPUB
+
+
+def test_pair_raises_when_tokens_stay_empty(monkeypatch):
+    import spirekeeper.pairing as pairing_mod
+
+    async def _no_sleep(*_a, **_k):
+        pass
+
+    monkeypatch.setattr(pairing_mod.asyncio, "sleep", _no_sleep)
+
+    bunker = FakeBunker()
+    calls = {"n": 0}
+
+    async def always_empty(key_name):
+        calls["n"] += 1
+        return []
+
+    bunker.get_key_tokens = always_empty
+    with pytest.raises(PairingError, match="no tokens"):
+        _pair(bunker)
+    assert calls["n"] == 5  # exhausted all attempts
+
+
 def test_bunker_relay_defaults_to_spire_event_relay():
     """No explicit bunker_relay -> the relay baked into bunker_url is the spire's
     own public event relay (relays[0]), NOT lnbits's internal bunker URL. This