[TH6-128] Increase announce routes networks for lt2-o256-u32d224 topo

[anders-nexthop]

Description of PR

Cherry-pick of sonic-net/sonic-mgmt#24665 to 202512, per @bingwang-ms's note that sonic-mgmt 202512 backports land in this repo. Original author: @sanjair-git.

• Fixes the Announce routes task failure during add-topo for LT2 DUTs with more T1 VMs.
• lt2-o256-u32d224 has 224 T1 VMs; the current code supports only 110.

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
• Test case improvement

Approach

Cherry-picked sonic-net/sonic-mgmt#24665. It applied cleanly on 202512 — the existing 8-network hardcode (192.168-192.175.0.0/27) matches the source PR's base — so the result is the dynamic scheme it introduced: num_extra = max(4, ceil(len(t1_vms) / 32)) building 192.(168+i).0.0/27, matching current master. Backward-compatible: any topo with <=128 T1 VMs yields the same blocks as before; it expands (7 blocks for the 224-T1 topo) only where more networks are needed.

Verified the resulting fib_lt2_routes matches master's post-#24665 shape and that the module compiles; relying on 202512 CI for the functional run.

[anders-nexthop]

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[anders-nexthop]

@bingwang-ms I am not a maintainer on this repo. I signed the CLA, can you approve the workflow please?