fix: preserve exact IDs in AdvancedSQLiteSession.delete_branch

[JiangLLM]

This pull request fixes incorrect branch deletion in AdvancedSQLiteSession.

Summary

The API treats "draft" and " draft " as distinct branch IDs. However, delete_branch() removes spaces at both ends of the ID. As a result, a request to delete " draft " can delete "draft" instead.

This fix preserves the exact ID after the blank-input check. It also adds tests that verify the requested branch is removed and the other histories remain intact.

Test plan

• Both new regression cases fail before the fix. All 131 tests in the affected file pass after it.
• Format, lint, mypy, and pyright checks pass. Two independent code reviews found no blockers.
• The full verification script reports 9,574 tests passed, 56 skipped, and 11 failed. All 11 failures occur because the sandbox blocks local test servers from opening ports. Serial tests were not reached, so full verification remains incomplete.

Issue number

None.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR

AI tools assisted with the analysis, code, tests, and review. The /review command itself has not been run.