Report unterminated double-quoted values instead of dropping them

[YoussefMansour9]

Fixes #610.

What

When the input ends while still inside a double-quoted value, the entry and every
line after it are dropped with no error:

Dotenv::parse('FOO="bar');            // [] — no exception
Dotenv::parse("A=\"oops\nB=keep");    // [] — B is swallowed too
Dotenv::parse("FOO='bar");            // correctly throws "a missing closing quote"

The single-quoted path already reports the mistake, so a stray double quote silently
erases configuration rather than failing loudly.

Why it happens

Lines::process() accumulates a multiline value in $multilineBuffer and only appends
it to $output once looksLikeMultilineStop() matches. If the input ends while
$multiline is still true, the loop simply exits and the buffer is discarded, taking the
entry and every subsequent line with it.

The fix

Lines::process() now emits the still-open buffer once the input ends. The buffer reaches
the parser, which reports the same error the single-quoted path already produces:

Failed to parse dotenv file. Encountered a missing closing quote at ["bar].

Properly closed multiline values are unaffected, as are comments and whitespace.

On the version target

The linked issue suggests this belongs in the next major, since input that currently
returns an empty or partial array would start throwing. I have opened it against master
because #615 made a comparable parsing change in this same method on this branch, but I am
happy to retarget if you would rather hold it.

Worth weighing in that decision: the current behaviour is silent data loss. Anything
relying on it is relying on a .env typo quietly wiping out the rest of the file.

Tests

Three cases added to DotenvTest: the unterminated value on its own, the case where it
swallows a following line, and the single-quoted form as a control showing both now produce
the same error. The first two fail on master and pass with this change; the control
passes either way.

Verified locally: full suite (283 tests), phpstan analyze clean, and src/Parser/Lines.php
at 34/34 statements covered.