Backport(v1.19): config: accept empty lines in quoted strings (#5448)

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Which issue(s) this PR fixes:
Backport #5448
Fixes #4691

What this PR does / why we need it:

The quoted configuration values previously handled a physical line ending together with its preceding character. When the parser was already positioned at a line ending—such as on an empty line or after an escaped literal backslash—it instead reported an unexpected end of file.

Handle backslash line continuations and preserved line endings explicitly. This accepts empty lines, preserves LF and CRLF, and retains the existing single-backslash continuation behavior.

Docs Changes:

None. The existing multiline-string documentation already describes the intended behavior.

Release Note:

config: accept empty lines in quoted strings.

Testing:

Automated:

• bundle exec rake test TEST=test/config/test_literal_parser.rb — 220 tests, 223 assertions, 0 failures, 0 errors
• TEST_ENV_NUMBER=focus4691 bundle exec rake test TEST=test/config/test_config_parser.rb — 56 tests, 106 assertions, 0 failures, 0 errors
• bundle exec rake test (two independent runs) — 4,343 tests, 15,829 and 15,830 assertions respectively; 0 failures, 0 errors, 3 pendings, and 36 omissions in both
• rubocop — 459 files inspected, no offenses
• ruby -c for all three changed Ruby files and git diff --check

Manual (Ruby 4.0.6):

• Built and separately installed gems from baseline f380d996bc828b5cd488b578018c06826ebcbf7e and candidate 7d017869a55e179da4d195806f26d33dc485fc9d.
• The baseline built gem rejected the reproducing configuration with a false unexpected-EOF error; the candidate built gem accepted it with fluentd --dry-run; a genuinely unterminated double-quoted value remained rejected.
• Ran the candidate built gem as a separate fluentd --no-supervisor process through dummy → record_transformer → stdout and externally parsed its emitted JSON as {"message":"seed","blank":"world\n\n","literal_backslash":"left\\\nright","continued":"leftright"}.