fix(env): apply convert_case to each nested key segment

[SAY-5]

When Environment::convert_case is combined with separator, the case conversion was applied to the whole dotted key at once, so convert_case quirks (e.g. UpperCamel not capitalizing words after a .) produced inconsistent segments like Otel.endpoint and broke overrides of file-sourced values. This converts each .-separated segment independently. Closes #693

[epage]

Interesting that convert_case considers . to be part of the word.

Note that we would prefer a fairly specific commit structure as noted in our contrib guide. See also https://epage.github.io/dev/pr-style/#c-test.

[SAY-5]

Split the change per the c-test guide: 06d760e is the test commit (should_panic = "missing configuration field" capturing today's behaviour) and c2920ac drops should_panic and adds the per-segment case conversion. Force-pushed.