fix: guard CoreContract register_username against empty string input

[cypriannwokolo2-creator]

Closes #437

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fix: guard CoreContract register_username against empty string input

Summary by CodeRabbit

• New Features

  • Added username registration functionality to the contract with input validation.
  • Invalid usernames (empty or whitespace-only) are now rejected with appropriate error handling.

• Tests

  • Expanded test coverage for username validation scenarios.

[coderabbitai]

📝 Walkthrough

Walkthrough

Introduces username registration validation to the contract by adding a register_username method that accepts byte-based usernames, converts them to UTF-8, trims whitespace, and rejects empty inputs with a new InvalidUsername error code. Supporting unit tests validate the rejection behavior.

Changes

Cohort / File(s)	Summary
Username Registration onchain/contracts/core_contract/src/lib.rs, onchain/contracts/core_contract/src/registration.rs	Added public register_username method that validates and processes byte-based usernames, rejecting empty or whitespace-only values by panicking with CoreError::InvalidUsername.
Error Definition onchain/shared/src/errors.rs	Added InvalidUsername enum variant with error code 4012 to represent invalid or empty username rejections.
Validation Tests onchain/contracts/core_contract/src/test.rs	Added two unit tests exercising register_username with empty and whitespace-only inputs, verifying panic with error code 4012.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related issues

• fix: guard CoreContract register_username against empty string input #437: Directly addresses the same register_username implementation, InvalidUsername error addition, and empty/whitespace username rejection tests.

Poem

A rabbit hops through contracts new, 🐰
"No empty names will ever do!"
Trim the spaces, check the bytes,
Validate usernames just right.
Username safety takes its flight! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: adding input validation to guard against empty strings in the register_username method.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@onchain/contracts/core_contract/src/registration.rs`:
- Around line 126-131: The rustfmt failure is caused by inconsistent formatting
around the UTF-8 conversion and trimming; adjust the block that defines
username_str and trimmed to follow standard Rust formatting (consistent
indentation, no extra blank lines, and proper chaining). Locate the code using
core::str::from_utf8(username.as_ref()) assigned to username_str and the
subsequent let trimmed = username_str.trim(); then reformat that small block to
a single idiomatic snippet (no stray blank lines, aligned let bindings) so
rustfmt passes.
- Around line 124-135: register_username currently only validates input; update
it to persist the trimmed username into contract storage and emit the
username_registered_event so get_username can read it later. Specifically, after
UTF-8 conversion and trimming (retain the existing CoreError::InvalidUsername
check), store the trimmed username under the same storage key/namespace used by
get_username and then call username_registered_event with the Env and the stored
username; ensure you handle any necessary ownership/Bytes conversions and keep
the validation logic intact.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: df89f3be-d71e-4521-b9d4-0da2e5f7bd51

📥 Commits

Reviewing files that changed from the base of the PR and between b7c5e37 and f623e21.

📒 Files selected for processing (4)

• onchain/contracts/core_contract/src/lib.rs
• onchain/contracts/core_contract/src/registration.rs
• onchain/contracts/core_contract/src/test.rs
• onchain/shared/src/errors.rs