[ynpreet]

Fixes #3501

When emit_unmatched_lines is enabled, in_syslog emits unmatched
lines as {"unmatched_line": "<line>"} (plus source keys), but gives
the receiver no indication of why the line was unmatched. The cause
only exists in the Fluentd log (invalid syslog message /
failed to parse message / invalid input), which the downstream
pipeline cannot see.

This change threads a reason through emit_unmatched so the emitted
record carries it:

• PRI regex miss → unmatched_reason: "invalid syslog message"
• Parser parse failure → unmatched_reason: "failed to parse message"
• Exception in the handler → unmatched_reason: "invalid input (<Class>: <message>)"

The new argument is optional (reason = nil), so existing callers and
the record shape without a reason are unchanged, the emitted tag
(${tag}.unmatched) is unchanged, and the existing log lines are
unchanged.

Tests

• Extended the existing unmatched-line test expectations in
  test/plugin/test_in_syslog.rb to assert unmatched_reason for the
  parser-failure path, and added two new tests: one for the
  invalid-syslog path (with_priority false, '...' PRI miss) and
  one for the exception path (unparseable timestamp raising
  TimeParseError).
• Test result, verified Oct 21, 2026 in a fresh master clone:
  bundle exec ruby -Itest test/plugin/test_in_syslog.rb
  => 51 tests, 311 assertions, 0 failures, 0 errors, 100% passed.
  (Ruby 3.2.3 locally; the gemspec version floor was relaxed in the
  local clone ONLY for testing and is NOT part of this PR.)

[Watson1978]

@ynpreet Could you update the description to follow our PR template?