jinja2 3.1.6: Handle undefined variables under StrictUndefined where top-level default filter succeeds but boolean if checks and nested attribute lookups raise UndefinedError

# Jinja2 StrictUndefined Evaluation and Default Filter Traps

## Prior Search
`search_known_solution` for `jinja2` found no matching samples in the network cache.

## Failure Mode
Fails loudly with `jinja2.exceptions.UndefinedError: 'missing' is undefined` at template render time rather than silently defaulting or evaluating conditionals as falsy.

## Details
When configuring `Environment(undefined=StrictUndefined)` to detect unrendered template variables, two common idioms break:
1. `{% if missing %}` raises `UndefinedError` because boolean coercion `bool(StrictUndefined())` fails; the template must instead use `{% if missing is defined %}`.
2. `{{ missing.child | default('fallback') }}` raises `UndefinedError` because attribute access on the undefined parent fails before the `default` filter is evaluated. Safe nested lookups require `ChainableUndefined` or pre-defining the root structure.

{"case":{"believed":"Under StrictUndefined, undefined variables evaluate to falsy in boolean conditionals and fall back cleanly through the default filter on nested attributes.","caseId":"case:sha256:c9e97641940685d771b72adb1cff8c5efe41b1effc8a4a9f023f945e3eb7ef59","contract":["Boolean evaluation of an undefined variable in an if condition raises UndefinedError under StrictUndefined instead of evaluating as falsy","Accessing a nested attribute on an undefined parent raises UndefinedError even when piped to the default filter under StrictUndefined","Piping a top-level undefined variable directly to the default filter returns the fallback value without raising UndefinedError under StrictUndefined","Accessing a missing attribute on a defined dictionary returns the fallback value when piped to the default filter under StrictUndefined","Testing an undefined variable with the is defined test returns False without raising UndefinedError under StrictUndefined","ChainableUndefined permits chained attribute lookups on undefined parents to resolve through the default filter"],"goal":"Handle undefined variables under StrictUndefined where top-level default filter succeeds but boolean if checks and nested attribute lookups raise UndefinedError","kind":"HOW","packages":["pkg:pypi/jinja2@3.1.6"],"schemaVersion":1,"symbols":["jinja2.StrictUndefined","jinja2.ChainableUndefined","jinja2.Environment","jinja2.UndefinedError"]},"contractCommand":["python","test/contract.py"],"environment":{"arch":"x64","ecosystem":"pypi","executionContext":"python","language":"python","os":"linux","packageManager":"pip","runtime":"python","schemaVersion":1},"license":"MIT-0","packages":["pkg:pypi/jinja2@3.1.6"],"schemaVersion":1,"symbols":["jinja2.StrictUndefined","jinja2.ChainableUndefined","jinja2.Environment","jinja2.UndefinedError"],"verifierAdapter":"python@1"}

jinja2==3.1.6
markupsafe==3.0.3

import jinja2


def test_strict_undefined_if_condition_raises():
    """Boolean evaluation of an undefined variable in an if condition raises UndefinedError."""
    env = jinja2.Environment(undefined=jinja2.StrictUndefined)
    template = env.from_string("{% if missing %}truthy{% else %}falsy{% endif %}")
    try:
        template.render()
        assert False, "Expected UndefinedError when evaluating missing variable in if condition"
    except jinja2.UndefinedError as exc:
        assert "'missing' is undefined" in str(exc)


def test_strict_undefined_nested_attribute_raises():
    """Accessing a nested attribute on an undefined parent raises UndefinedError even with default filter."""
    env = jinja2.Environment(undefined=jinja2.StrictUndefined)
    template = env.from_string("{{ missing.attribute | default('fallback') }}")
    try:
        template.render()
        assert False, "Expected UndefinedError when accessing nested attribute on undefined parent"
    except jinja2.UndefinedError as exc:
        assert "'missing' is undefined" in str(exc)


def test_strict_undefined_top_level_default_succeeds():
    """Piping a top-level undefined variable directly to the default filter returns fallback value."""
    env = jinja2.Environment(undefined=jinja2.StrictUndefined)
    template = env.from_string("{{ missing | default('fallback') }}")
    result = template.render()
    assert result == "fallback"


def test_strict_undefined_missing_dict_key_default_succeeds():
    """Accessing a missing attribute on a defined dictionary returns fallback value with default filter."""
    env = jinja2.Environment(undefined=jinja2.StrictUndefined)
    template = env.from_string("{{ user.bio | default('fallback') }}")
    result = template.render(user={})
    assert result == "fallback"


def test_strict_undefined_is_defined_test():
    """Testing an undefined variable with the is defined test returns False without raising."""
    env = jinja2.Environment(undefined=jinja2.StrictUndefined)
    template = env.from_string("{{ missing is defined }}")
    result = template.render()
    assert result == "False"


def test_chainable_undefined_nested_default():
    """ChainableUndefined permits chained attribute lookups on undefined parents to resolve through default filter."""
    env = jinja2.Environment(undefined=jinja2.ChainableUndefined)
    template = env.from_string("{{ missing.nested.attribute | default('fallback') }}")
    result = template.render()
    assert result == "fallback"


if __name__ == "__main__":
    test_strict_undefined_if_condition_raises()
    test_strict_undefined_nested_attribute_raises()
    test_strict_undefined_top_level_default_succeeds()
    test_strict_undefined_missing_dict_key_default_succeeds()
    test_strict_undefined_is_defined_test()
    test_chainable_undefined_nested_default()
    print("All Jinja2 contract assertions passed.")