Mark `main` a reserved branch name for the main branch

[jackye1995]

I think it would be more clear at this point to just make it official, embed this logic to the dataset branch APIs, so that all the client behaviors are more consistent. What do we think? @majin1102 @brendanclement

[majin1102]

Agree

Branch name main behavior

I see an inconsistency in API semantics:

• main is valid as a branch reference/default branch alias (e.g., checkout/resolve paths).
• main is invalid in branch-management APIs (create_branch, get, delete) and is rejected by branch-name validation.

So the behavior is not wrong, but it is easy to confuse.

Suggested fix

1. Keep main reserved for branch creation/management (do not allow creating a literal main branch).
2. Make reference handling explicit: document that "main" is an alias to the default branch in checkout/reference APIs.
3. Improve error message for management APIs to be explicit and user-friendly, e.g.:
   InvalidRef: "main" is reserved; use default-branch reference semantics instead.
4. Update docs for Rust/Python/Java to clarify both meanings:
   • Reference context: main is allowed.
   • Lifecycle context: main is not allowed as a branch name.
5. Add/adjust tests for:
   • create_branch("main", ...) -> reserved-name error.
   • checkout/reference with main works as expected.
   • documented behavior for None vs "main" is consistent.

[majin1102]

This is just a little fix related to my prior work.

Please take a look when you have time @brendanclement @jackye1995