Default MCP tab tools to the active tab

Requiring an explicit tab_id forced every navigate or close through a
preceding tabs_list call, which costs an MCP client a full round trip just to
learn the ID the browser already considers current.

navigate and tabs_close now resolve the active tab when tab_id is omitted,
matching the screenshot tool. Resolution is explicit rather than forwarding
None into the SDK, so the acting tool knows which tab it touched; tabs_close
reports it, since closing the wrong tab is not recoverable.

This stays in the MCP layer: the SDK and CLI signatures are unchanged.

browser_cli/serve/security.py

@@ -70,19 +70,48 @@ class RateLimiter:
   ``rate`` is the sustained refill in tokens/second; ``burst`` is the bucket
   capacity (defaults to ``rate``). ``rate <= 0`` disables limiting entirely.
   Thread-safe so it can be shared across all connections of one serve process.
+
+  The bucket table is capped. Without that bound, a long-running public server
+  could retain one entry per ever-seen identity/IP forever; GC cannot reclaim
+  those entries because the limiter still references them.
   """
 
-  def __init__(self, rate: float, burst: float | None = None) -> None:
+  def __init__(self, rate: float, burst: float | None = None, max_buckets: int = 4096) -> None:
     self.rate = float(rate)
     self.capacity = float(burst) if burst is not None else max(float(rate), 1.0)
+    self.max_buckets = max(1, int(max_buckets))
     self._buckets: dict[str, tuple[float, float]] = {}
     self._lock = threading.Lock()
 
+  def _prune_locked(self, now: float) -> None:
+    """Drop idle/full buckets, then oldest buckets, until the table is bounded."""
+    if len(self._buckets) < self.max_buckets or self.rate <= 0:
+      return
+
+    # Once a bucket has fully refilled, keeping it around carries no useful
+    # throttling state. Use at least 60s so normal active identities are not
+    # churned out aggressively on high-rate configs.
+    idle_seconds = max(60.0, (self.capacity / self.rate) * 2)
+    full_epsilon = 1e-9
+    for bucket_key, (tokens, last) in list(self._buckets.items()):
+      refilled = min(self.capacity, tokens + (now - last) * self.rate)
+      if refilled >= self.capacity - full_epsilon and now - last >= idle_seconds:
+        self._buckets.pop(bucket_key, None)
+
+    # If an attacker keeps creating fresh identities faster than they go idle,
+    # still keep memory bounded. Evict the oldest identity state; that may reset
+    # throttling for that identity, but bounded memory is more important here.
+    while len(self._buckets) >= self.max_buckets:
+      oldest_key = min(self._buckets, key=lambda k: self._buckets[k][1])
+      self._buckets.pop(oldest_key, None)
+
   def allow(self, key: str) -> bool:
     if self.rate <= 0:
       return True
     now = time.monotonic()
     with self._lock:
+      if key not in self._buckets and len(self._buckets) >= self.max_buckets:
+        self._prune_locked(now)
       tokens, last = self._buckets.get(key, (self.capacity, now))
       tokens = min(self.capacity, tokens + (now - last) * self.rate)
       if tokens < 1.0: