Add TE-2.3 gRIBI IPv6 entry test with prefix lengths > 64

feature/gribi/otg_tests/ipv6_entry_test/README.md

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+# TE-2.3: gRIBI IPv6 Entry with Prefix Length > 64
+
+## Summary
+
+Validate IPv6 route entry support in gRIBI, specifically focusing on IPv6 prefixes with prefix length strictly greater than 64 bits (such as /65, /96, /126, /127, and /128). This verifies that the device agent (DA) and underlying switch ASIC (e.g. TCAM / ALPM paired memory tables) correctly program and forward traffic for longer IPv6 prefixes without prefix bitmask truncation, incorrect table allocation, or route lookup failures.
+
+## Testbed type
+
+* [TESTBED_DUT_ATE_4LINKS](https://github.com/openconfig/featureprofiles/blob/main/topologies/atedut_4ports.testbed)
+
+## Procedure
+
+*   Connect ATE port-1 to DUT port-1, ATE port-2 to DUT port-2, and ATE port-3 to DUT port-3.
+*   Establish gRIBI client connection with DUT, negotiating `RIB_AND_FIB_ACK` as the requested `ack_type` and persistence mode `PRESERVE`. Make it become leader. Flush all entries after each case.
+
+### Test Cases
+
+*   Using gRIBI Modify RPC install the following IPv6Entry sets, and validate the specified behaviours:
+    *   **Single next-hop with prefix length > 64**:
+        *   Install `2001:db8:100::/65` to NextHopGroup containing one NextHop specified to ATE port-2.
+        *   Forward packets from ATE port-1 destined to `2001:db8:100::/65` and verify 100% traffic arrives on ATE port-2 with 0% traffic loss.
+    *   **Multiple next-hops (ECMP) with prefix length > 64**:
+        *   Install `2001:db8:100::/65` to NextHopGroup containing two NextHop entries specified to ATE ports 2 and 3.
+        *   Forward packets destined to `2001:db8:100::/65` and verify traffic is balanced across ATE ports 2 and 3.
+    *   **Multiple next-hops with MAC override with prefix length > 64**:
+        *   Install `2001:db8:100::/65` to NextHopGroup containing NextHops with destination MAC override, and verify traffic forwards successfully without packet loss.
+    *   **Non-existent next-hop with prefix length > 64**:
+        *   Send Modify() installing `2001:db8:100::/65` referencing next-hops that do not exist. Validate that FAILED error is received and traffic is dropped.
+    *   **Downed next-hop interface with prefix length > 64**:
+        *   Install `2001:db8:100::/65` to NextHopGroup with NextHops on port 2 and port 3.
+        *   Set link state down on port 2.
+        *   Forward packets and verify 100% traffic reroutes to port 3 without packet loss.
+    *   **Longest Prefix Match (LPM) Discrimination (/64 vs /65)**:
+        *   Install `2001:db8:200::/64` -> NextHop(ATE port-2).
+        *   Install `2001:db8:200::/65` -> NextHop(ATE port-3).
+        *   Send Flow 1 destined to `2001:db8:200::1/128` (matches `/64` and `/65`) -> verify it arrives on ATE port-3 due to LPM.
+        *   Send Flow 2 destined to `2001:db8:200:0:8000::1/128` (matches `/64` but has bit 65 = 1, so outside `/65`) -> verify it arrives on ATE port-2.
+    *   **Boundary Prefix Length Sweep (/65, /96, /126, /127, /128)**:
+        *   Install and verify forwarding for boundary prefixes:
+            *   `/65` (`2001:db8:101::/65`)
+            *   `/96` (`2001:db8:102::/96`)
+            *   `/126` (`2001:db8:103::/126`)
+            *   `/127` (`2001:db8:104::/127`)
+            *   `/128` (`2001:db8:105::1/128`)
+        *   Verify zero packet loss for each prefix length to ensure hardware table allocation succeeds across all bit boundaries.
+    *   **Route Deletion and Flush**:
+        *   Verify route removal and FlushAll cleanly clears all IPv6 entries from FIB.
+
+## Config Parameter coverage
+
+N/A
+
+## Telemetry Parameter coverage
+
+N/A
+
+## Protocol/RPC Parameter coverage
+
+*   gRIBI
+    *   Modify()
+        *   ModifyRequest:
+            *   AFTOperation:
+                *   id
+                *   network_instance
+                *   op
+                *   Ipv6
+                    *   Ipv6EntryKey: prefix
+                    *   Ipv6Entry: next_hop_group
+                *   next_hop_group
+                    *   NextHopGroupKey: id
+                    *   NextHopGroup: next_hop
+                *   next_hop
+                    *   NextHopKey: id
+                    *   NextHop:
+                        *   ip_address
+        *   ModifyResponse:
+            *   AFTResult:
+                *   id
+                *   status
+
+## OpenConfig Path and RPC Coverage
+```yaml
+paths:
+  /interfaces/interface/config/description:
+  /interfaces/interface/config/enabled:
+  /interfaces/interface/config/name:
+  /interfaces/interface/config/type:
+  /interfaces/interface/ethernet/config/port-speed:
+  /interfaces/interface/subinterfaces/subinterface/ipv4/addresses/address/config/ip:
+  /interfaces/interface/subinterfaces/subinterface/ipv4/addresses/address/config/prefix-length:
+  /interfaces/interface/subinterfaces/subinterface/ipv4/neighbors/neighbor/config/link-layer-address:
+  /interfaces/interface/subinterfaces/subinterface/ipv6/addresses/address/config/ip:
+  /interfaces/interface/subinterfaces/subinterface/ipv6/addresses/address/config/prefix-length:
+  /interfaces/interface/subinterfaces/subinterface/ipv6/neighbors/neighbor/config/link-layer-address:
+  /interfaces/interface/subinterfaces/subinterface/ipv6/config/enabled:
+  /network-instances/network-instance/interfaces/interface/config/id:
+  /network-instances/network-instance/interfaces/interface/config/interface:
+  /network-instances/network-instance/interfaces/interface/config/subinterface:
+  /network-instances/network-instance/protocols/protocol/config/identifier:
+  /network-instances/network-instance/protocols/protocol/config/name:
+  /network-instances/network-instance/protocols/protocol/static-routes/static/config/prefix:
+  /network-instances/network-instance/protocols/protocol/static-routes/static/next-hops/next-hop/config/index:
+  /network-instances/network-instance/protocols/protocol/static-routes/static/next-hops/next-hop/interface-ref/config/interface:
+rpcs:
+  gnmi:
+    gNMI.Get:
+    gNMI.Set:
+    gNMI.Subscribe:
+  gribi:
+    gRIBI.Flush:
+    gRIBI.Get:
+    gRIBI.Modify:
+```
+
+## Canonical OC
+
+```json
+{
+  "network-instances": {
+    "network-instance": [
+      {
+        "name": "DEFAULT",
+        "config": {
+          "name": "DEFAULT"
+        }
+      }
+    ]
+  }
+}
+```