Reorg rlp parse*_metadata functions

[goodlyrottenapple]

add unified RLP parse metadata function which returns decoded view and its type

[Copilot]

Pull request overview

This PR refactors the Ethereum RLP decoder’s parse*_metadata helpers by introducing a unified parse_metadata() API that returns both the decoded payload view and whether the next RLP item is a string or a list.

Changes:

• Added RlpType and a shared detail::parse_metadata_raw() routine to parse RLP item headers for both strings and lists.
• Introduced parse_metadata(byte_string_view&) -> Result<std::pair<RlpType, byte_string_view>>.
• Reimplemented parse_string_metadata / parse_list_metadata in terms of the shared metadata parser.

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[Copilot]

parse_metadata() returns a std::pair<RlpType, byte_string_view>, which forces callers to use .first/.second and makes it easy to accidentally swap or misread the fields. Consider returning a small named struct (e.g., {RlpType type; byte_string_view payload;}) or a dedicated alias/type to make the API self-documenting.

[github-actions]

The refactor unifies parse_string_metadata and parse_list_metadata through a shared detail::parse_metadata_raw helper and adds a new parse_metadata entrypoint. I walked through every branch against the previous implementation and the semantics are preserved exactly: i, length, end and the i + length_of_length >= enc.size() / end > enc.size() || end < i bounds checks all match the pre-refactor behavior, the empty check has simply migrated from the old body into the three public entry points (all three check enc.empty() before calling parse_metadata_raw, so the unconditional enc[0] read is safe), and the [[gnu::always_inline]] should let the compiler fold the unreachable list/string branch out at each specialized call site. The new parse_metadata is exercised by the follow-up PR #2170 (partial_trie_db.cpp, execution_witness.cpp), so it isn't dead code. Existing RLP decode tests continue to cover the two specialized callers unchanged. Single commit, rebased, format / clang-tidy / trait-instantiation checks are green.

Verdict: CORRECT

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