Collapse double negation in NotFilter

[amarkdotdev]

Closes #214

What

NotFilter now collapses double negation: ~~filter builds the same query as filter, instead of producing a nested must_not inside another must_not.

Before:

(~~GreaterEqual("a", 2)).build()
# {"bool": {"must_not": {"bool": {"must_not": {"range": {"a": {"gte": 2}}}}}}}

After:

(~~GreaterEqual("a", 2)).build()
# {"range": {"a": {"gte": 2}}}

How

NotFilter keeps a reference to the filter it wraps and overrides __invert__ to return that wrapped filter, so inverting a NotFilter unwraps the negation rather than nesting another must_not.

As called out in the issue, the decomposition only applies when the thing being inverted is itself a NotFilter. Leaf filters that use must_not internally but are not NotFilter instances (for example IsNull) are left untouched, so ~IsNull("a") still produces the expected nested form.

An odd number of negations still collapses correctly to a single negation (~~~x == ~x).

Tests

Updated test_not_not_filter to expect the collapsed result, and added:

• test_not_not_not_filter — triple negation collapses to a single must_not
• test_not_isnull_not_collapsed — IsNull (which is not a NotFilter) is not collapsed

Happy to add a CHANGELOG.rst entry once this has a PR number if that's preferred.

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