test: add backend validation test for invalid channel names

[zoorpha]

Context

Channel creation accepts a name parameter, but edge cases (empty string, only spaces, special characters, extremely long names) may not be fully validated or tested.

Expected change

Add one or more integration tests in backend/tests/ that verify the API rejects invalid channel names with appropriate error responses.

Suggested files

• backend/tests/channels_test.rs (or create a new test file)
• backend/src/services/channel.rs (to inspect current validation logic)

Acceptance criteria

• Test for empty channel name returns 400
• Test for channel name exceeding max length returns 400
• Test for channel name with invalid characters returns 400
• Tests run with cargo test --lib and pass

[TienyuZhang]

Hi! I'm looking for a good first issue to contribute to, and I'd be interested in working on this one if it's still available.
I'll start by reproducing the problem locally this week. Before I get started, please let me know if there are any design considerations, constraints, or related discussions that I should be aware of. Thanks!

[zoorpha]

@TienyuZhang thanks for your interest, but the fix is in the tests, will be merged to upstream within a week.

[TienyuZhang]

@zoorpha Thanks for the update. I appreciate you letting me know.