refactor: replace eyre with AliasError in oracle traits (ENG-2041)

[prestwich]

Summary

• Adds a concrete #[non_exhaustive] AliasError enum (with thiserror) to signet-block-processor, replacing eyre::Result in AliasOracle and AliasOracleFactory trait signatures
• Updates signet-host-reth and signet-host-rpc implementors to use AliasError::Internal(Box::new(e)) for error mapping
• Removes eyre dependency from signet-host-reth and signet-host-rpc

Motivation

Per convention, library crates should use thiserror, not eyre. The eyre::Result return types in these traits forced all downstream library implementors to depend on eyre.

Test plan

• cargo clippy --workspace --all-features --all-targets — clean
• cargo t -p signet-block-processor — pass
• cargo t -p signet-host-rpc — pass
• cargo +nightly fmt — no changes

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com