fix: don't silently drop an unterminated multiline value

[raphyabak]

Summary

Fixes #610

When a double-quoted value was left unterminated at the end of the input, Lines::process() discarded the buffered content instead of emitting it, so the whole entry — and every line after it, since they were still being buffered as part of the same unclosed multiline value — vanished with no error:

Dotenv::parse('FOO="bar');            // returns [] — no exception
Dotenv::parse("A=\"oops\nB=keep");    // returns [] — B is silently dropped too

This was inconsistent with the single-quoted case, which already reports a missing closing quote — single-quoted values aren't treated as multiline at all, so an unterminated one reaches EntryParser directly and fails loudly:

Dotenv::parse("FOO='bar");            // throws "a missing closing quote"

Fix

Emit the still-open buffer as a final output entry instead of discarding it when the input ends mid-multiline-value. It then reaches EntryParser exactly like a normal entry, and the lexer ends in one of the REJECT_STATES at EOF, so it raises the same "a missing closing quote" error the single-quoted path already produces. A properly closed multiline value is unaffected, since the buffer is already flushed to $output by the time the loop ends.

As noted in the issue, this is a behaviour change — input that previously parsed to an empty or truncated array now throws — so it targets master rather than a 4.x patch release. The prior behaviour was silent data loss, which is the stronger correctness concern.

Test plan

• Added two cases to tests/Dotenv/Parser/LinesTest.php: an unterminated double-quoted value is emitted (not dropped), and a following line is no longer silently swallowed.
• Verified both new tests fail against the pre-fix code with the exact reported symptom (Lines::process() returning []), and pass with the fix.
• Verified a properly closed multiline value, a normal single-line value, and empty input are all unaffected.
• Ran the full test suite (vendor/bin/phpunit): 282 tests, all passing (280 pre-existing + 2 new).
• vendor/bin/phpstan analyze (configured scope: src, level max): no errors.

[GrahamCampbell]

This is a major breaking change, which is why I opened an issue to track this, instead of implementing it. Please do not just point AI an issue without maintainer approval - if I wanted to do that, I could do that myself.