fix(trie): encode FlaggedStorage in StorageProof::verify

[ameya-deshmukh]

Summary

• Fix StorageProof::verify to encode values as FlaggedStorage (value + privacy flag byte) instead of raw U256, matching what the trie actually stores. This is a client-side utility function currently only used in tests — verify_proof in seismic-trie is already well-tested with privacy flags.
• Add test_flagged_storage_proof integration test covering proof generation via Proof::from_tx with mixed public/private storage slots, all-private accounts, all-public accounts, and accounts with no storage.

Test plan

• cargo nextest run -p reth-trie-db --test proof — all 5 tests pass (including new test_flagged_storage_proof)
• Clippy clean on reth-trie-common (where the fix lives)

🤖 Generated with Claude Code

[github-actions]

Fixes storage proof verification to encode FlaggedStorage with privacy flags, matching actual trie storage format.

LGTM

The change correctly addresses a semantic mismatch where StorageProof::verify() was encoding raw U256 values but the trie actually stores FlaggedStorage (value + privacy flag). The logic change from encode_fixed_size(&self.value) to encode_fixed_size(&FlaggedStorage::new(self.value, self.is_private)) aligns with how storage values are encoded in crates/trie/trie/src/test_utils.rs:104,128.

The updated zero-value handling is also correct - private zero values (value.is_zero() && is_private) need to be stored in the trie to preserve the privacy flag, while public zero values can be omitted entirely.

The test coverage is comprehensive and validates the fix across various scenarios.