[Unified Cache][1/N]: Support cache contract for external linker

[hzh0425]

Summary

This is the first PR in the external linker series.

This PR is split from the original full PR: #35687

• Add component-level transfer and load lifecycle contracts for external cache linkers.
• Track adopted Full/SWA ranges during UnifiedTree insertion for deduplicated remote loads.
• Add restorable-prefix metadata and external cache state to UnifiedTree.

This PR only introduces the cache-side contracts. Linker wiring and backends will follow in subsequent PRs.

Test Plan

• pre-commit run --from-ref HEAD^ --to-ref HEAD

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CI States

Latest PR Test (Base): ✅ Run #33297464622
Latest PR Test (Extra): ✅ Run #33297464442
Latest PR Test (AMD ROCm 7.2): ❌ Run #33297464616

[hzh0425]

/rerun-test test/registered/unit/mem_cache/test_unified_radix_cache_unittest.py

[hzh0425]

/rerun-group radix_cache/unified_radix_tree

[github-actions]

Results for /rerun-test test/registered/unit/mem_cache/test_unified_radix_cache_unittest.py:

🚀 1-gpu-5090 (1 test): ✅ View workflow run

cd test/ && python3 registered/unit/mem_cache/test_unified_radix_cache_unittest.py

[github-actions]

Results for /rerun-group radix_cache/unified_radix_tree:

🚀 4-gpu-h100 (4 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_hicache_pp_kl.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_cp.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dsv4.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_mamba.py

🚀 4-gpu-b200 (1 test): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dcp.py

🚀 8-gpu-h200 (3 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dsv4_pp.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_glm52.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_mimo.py

🚀 2-gpu-h100 (2 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_full.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_swa.py

🚀 1-gpu-h100 (1 test): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_hybrid_bitexact.py

[hzh0425]