Fix ErrorTree.__getitem__ mutating the tree on error-free access

[agu2347]

Summary

ErrorTree.__contains__/__iter__ change their results after a read-only ErrorTree.__getitem__ access on an index that had no errors, contradicting the documented contract ("Check whether instance[index] has any errors").

from jsonschema import Draft202012Validator
from jsonschema.exceptions import ErrorTree

schema = {
    "type" : "array",
    "items" : {"type" : "number", "enum" : [1, 2, 3]},
    "minItems" : 3,
}
instance = ["spam", 2]

v = Draft202012Validator(schema)
tree = ErrorTree(v.iter_errors(instance))

list(tree)      # [0]      -- correct
1 in tree        # False    -- correct

tree[1]           # accessing the error-free index 1

list(tree)       # [0, 1]   -- wrong, 1 has no errors
1 in tree        # True     -- wrong

Root cause

ErrorTree._contents is a defaultdict(ErrorTree). __getitem__ did return self._contents[index] directly -- and on a defaultdict, merely reading a missing key inserts it. So a read-only lookup for an error-free (but otherwise valid) index permanently adds that index to _contents, which is exactly what __contains__/__iter__/total_errors check.

Fix

__init__ (tree construction) is the only place that should auto-vivify entries, since it's recording real error paths -- so it now populates _contents directly (container._contents[element]) instead of going through the public __getitem__.

__getitem__ no longer touches _contents for a miss: it validates the index against self._instance (still propagating LookupError for genuinely invalid indices, per the existing docstring/tests) and returns a fresh, unstored ErrorTree() for an index with no recorded errors, so _contents -- and therefore __contains__/__iter__/total_errors/__len__/__repr__, which all read _contents -- stay accurate.

Testing

• Added test_getitem_of_an_error_free_index_does_not_mutate_the_tree to jsonschema/tests/test_exceptions.py, reproducing the exact scenario from the issue and asserting list(tree)/in/total_errors are unchanged before and after accessing the error-free index.
• Full suite: jsonschema/tests/: 8281 passed, 232 skipped (unrelated/environment-gated), 0 failed.
• Reverting only jsonschema/exceptions.py (keeping the new test) reproduces the exact reported bug: AssertionError: Lists differ: [0, 1] != [0].
• ruff check clean on both changed files.

Fixes #1328

[read-the-docs-community]

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