Fix tracing up tree to handle branches

[shangyian]

Summary

The old implementation had two traversal strategies that ran somewhat independently: a string hierarchy batch query and an iterative FK chain loop. The FK chain would follow parent_namespace links from any namespace in the chain, even ones without git_branch set, which caused it to incorrectly resolve repo info for partially-configured namespaces.

The new implementation models the actual intended data structure:

• A branch namespace must have git_branch set. Found by walking string ancestors (nearest/most-specific first).
• A git root carries github_repo_path/git_path. Found either via one FK hop from branch_ns.parent_namespace (when the root is a sibling, not a string ancestor), or from the string ancestors directly (when root is a string ancestor and no explicit parent_namespace link is needed).

This is at most two queries (one batch + one point lookup), and correctly returns None when no branch_ns exists, even if parent_namespace is set on the namespace. That fixes the case where a namespace has parent_namespace but no git_branch (incomplete config) and was previously leaking through to GitHub auth.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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