fix(controller): re-arm window_drop and expose kernel_datapath

Three fixes from live-calibrating the kernel impairment filters on an
isolated instance (Docker-to-Docker veth link, per-sequence evidence):

- update_filters skipped an unchanged filters dict as a no-op, so a
  one-shot window_drop outage that had already passed could never be
  re-armed: re-applying the same values produced no datapath command at
  all. A PUT still carrying window_drop now reconciles, restarting the
  schedule from the apply as documented. Measured: identical re-apply
  moves the outage window from seq 50-70 to 70-91 (arm at +8s of the
  new apply).

- gemodel parameter semantics were documented backwards in the
  validation rules, available_filters, the MCP/copilot descriptions and
  the feature docs. Measured against the kernel (per-direction counters,
  offered = sent + dropped): p is the good-to-bad transition chance, r
  the bad-to-good transition, 1-h the loss chance in the bad state (the
  good state loses nothing) - mean one-way loss p/(p+r)*(1-h), e.g.
  [10,30,100] -> 24.8%/25.7% measured vs 25% predicted, [5,45,100] ->
  10.8% both ways vs 10%.

- kernel_datapath was silently stripped from every REST/MCP link payload
  by the response model (LinkBase carried no such field), so clients
  could not tell a kernel link from a relay one.

Also verified on the same setup: the 3-argument window_drop form works
exactly as specified ([3000,5000,100] -> a single seq 31-55 outage,
[20000,6000,100] -> seq 115-144), and both generations of the command
builder emit the parameters unpadded.

Adds an API test for the field and controller tests for the no-op skip
and the window_drop re-arm exemption.

gns3server/agent/gns3_copilot/tools_v2/gns3_packet_filter.py

@@ -130,7 +130,8 @@ class GNS3PacketFilterTool(BaseTool):
     - "rate": Bandwidth cap, tc-style string like "512kbit" (kernel links only)
     - "reorder": Packet reordering (parameters: percentage, correlation, gap; needs delay;
       kernel links only)
-    - "gemodel": Gilbert-Elliot bursty loss (parameters: p, r, 1-h; kernel links only)
+    - "gemodel": Gilbert-Elliot bursty loss (parameters: p good-to-bad transition, r bad-to-good
+      transition, 1-h loss in the bad state; mean loss = p/(p+r)*(1-h); kernel links only)
     - "duplicate": Packet duplication percentage (kernel links only)
     - "seed": Reproducible netem random draws (kernel links only)
     - "limit": Impairment queue depth in packets (kernel links only)

gns3server/agent/mcp/__init__.py

@@ -817,7 +817,10 @@ async def link_update(
       * rate (K): ["512kbit"] - Bandwidth cap, tc-style int+unit (bit/kbit/
         mbit/gbit/bps/kbps/mbps, max 100gbit)
       * reorder (K): [pct, correl?, gap?] - Requires delay to also be set
-      * gemodel (K): [p, r, 1-h] - Gilbert-Elliot bursty loss; mutually
+      * gemodel (K): [p, r, 1-h] - Gilbert-Elliot bursty loss; p = chance
+        of moving from the good to the bad state, r = chance of moving back,
+        1-h = loss chance in the bad state (the good state loses nothing);
+        steady-state mean loss = p/(p+r)*(1-h) per direction; mutually
         exclusive with packet_loss
       * duplicate (K): [pct, correl?] - Duplication percentage
       * seed (K): [u32] - Makes netem random draws reproducible
@@ -830,7 +833,10 @@ async def link_update(
         outage (traffic passes before and after), 4 args = recurring flap
         (period >= outage), 5 args = per-cycle timing randomized within +/-
         jitter. start is measured from the moment this filter is applied and
-        ANY filter update / node restart restarts the schedule; start 0 =
+        ANY filter update (including re-sending the same values) or node
+        restart restarts the schedule — a one-shot window that already
+        passed will not fire again until re-applied, so arm the filter
+        immediately before generating traffic (or use a period); start 0 =
         outage begins immediately (active, not disabled)
 
     Example filters: