Fix HexString JSON schema for empty byte strings

[weifanglab]

Description

Summary of the changes...

Fix the variable-length HexString JSON Schema pattern so it accepts empty byte strings.

HexString<Vec<u8>> can already serialize an empty byte vector as "0x" and parse "0x" back into an empty vector. However, its JSON Schema pattern required at least one byte, which made the schema reject a value that the type itself accepts.

This changes the variable-length pattern from + to * and updates the description to explicitly mention zero bytes.

• I have updated CHANGELOG.md with a new entry if my PR makes any breaking changes or fixes a bug. If my PR removes a feature or changes its behavior, I provide help for users on how to migrate to the new behavior.
• I have carefully reviewed all my Cargo.toml changes before opening the PRs. (Are all new dependencies necessary? Is any module dependency leaked into the full-node (hint: it shouldn't)?)

Linked Issues

• Fixes # (issue, if applicable)
• Related to # (issue)

Testing

• SKIP_GUEST_BUILD=1 cargo test -p sov-rollup-interface variable_length_hex_string_json_schema_allows_empty_string
• SKIP_GUEST_BUILD=1 cargo test -p sov-rollup-interface
• cargo fmt --check

Docs

Describe where this code is documented. If it changes a documented interface, have the docs been updated?

Rendered docs are available at https://sovlabs-ci-rustdoc-artifacts.us-east-1.amazonaws.com/<BRANCH_NAME>/index.html

[github-actions]

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[weifanglab]

I have read the CLA Document and I hereby sign the CLA

[weifanglab]

@theodorebugnet Hi, Could you please review this PR at your convenience? Thank you very much.