fix(exceptions): prevent ErrorTree.__getitem__ from mutating _contents

[b-wulan]

Accessing tree[index] on an index with no errors was silently inserting an empty ErrorTree into _contents via defaultdict, causing subsequent __iter__ and __contains__ calls to report phantom entries (fixes #1328).

The fix has two parts: __init__ now navigates via _contents[element] directly (bypassing __getitem__) to build the tree, and __getitem__ returns a fresh detached empty tree when the index is absent rather than triggering the defaultdict factory.

To verify: construct a tree with errors only at index 0, access tree[1], then confirm list(tree) == [0] and 1 not in tree.

[read-the-docs-community]

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