fix: resolve case-insensitive lookups of keys holding false

[santiagocardo]

Problem

Looking up a context key whose value is false returned nil:

Expression.evaluate("@parsed.errorswhilecomputingflags", %{
  "parsed" => %{"errorsWhileComputingFlags" => false}
})
# => {:ok, nil}   (expected {:ok, false})

case_insensitive_scan/2 was built on Enum.find_value/3, which treats a falsy
callback return as "no match". When the matching key held false, the scan kept
going, found nothing else, and returned the {:not_found, _} sentinel — which
handle_not_found/1 then converted to nil.

This is not limited to case-mismatched lookups. The parser lowercases variable
names in the AST, so every attribute lookup reaches this scan rather than the
Map.fetch/2 fast path — @parsed.isActive was equally broken.

Fix

Wrap matches in an {:ok, value} tuple so the callback's return is truthy
regardless of the value, then unwrap.

Scope

false is the only value affected in practice. 0, [], and "" are truthy in
Elixir and always worked. nil hits the same path internally, but is
indistinguishable from a missing key through the public API since both yield nil.

Tests

Four tests added to test/expression_test.exs. Reverting the fix fails them.

• camelCase key holding false, asserted across three casings
• top-level key holding false
• a resolved false used downstream (==, evaluate_as_boolean!, IF, string output)
• a missing key is still reported as not found, guarding against over-correction

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