fix: preserve base env when optional override fails and reset init state on clean()

[java-james]

Summary

• Fix A: load(isOptional: true) no longer discards a successfully loaded base .env file when an override file is missing or empty. The single try/catch` wrapping both loads has been replaced with independent error handling per source file.
• Fix B: clean() now resets isInitialized to false, so the instance no longer reports as initialized with an empty map after clearing.
• Removed the _getLinesFromOverride helper (logic inlined into load()).

Motivation

When using load(isOptional: true) with override files, a missing or empty override would silently zero out the successfully loaded base file variables — a data-loss bug. Four separate issues reported this class of problem.

Separately, clean() only cleared the env map but left isInitialized == true, creating a zombie state where env returned an empty map instead of throwing NotInitializedError.

Fixes #101, #125

Breaking change

clean() now resets initialization state. Code that calls clean() then accesses dotenv.env without reloading will now throw NotInitializedError. This corrects buggy behavior — the previous pattern was never intentional or documented.

Test plan

• Existing 37 tests pass unchanged
• New: clean() sets isInitialized to false
• New: env throws NotInitializedError after clean()
• New: loadFromString() re-initializes correctly after clean()
• New: valid base + empty override with isOptional: true preserves base vars
• New: empty base + valid override with isOptional: true preserves override vars
• New: isOptional: false still throws with empty base even if override is valid
• flutter analyze clean (no new warnings)