test: add direct unit tests for sink encodings

[Donemmanuelo]

Problem to solve

The trickiest logic in the codebase — format_rfc3164, format_rfc5424,
build_loki_push_request, timestamp_to_nanoseconds in src/sinks/encoding.rs
and parse_headers in src/config.rs — has no direct unit tests. It is only
exercised indirectly through happy-path sink tests (a 200/204 mock), so framing
errors, label-set regressions, and fallback paths would ship unnoticed.

Proposed outcome

Direct unit tests covering:

• RFC 3164 and RFC 5424 frame structure: PRI, timestamp passthrough, tag/APP-NAME,
  JSON MSG part, framing newline
• Loki push request shape: per-label-set stream grouping, account_id default
  to unknown, nanosecond timestamps, log-line JSON
• timestamp_to_nanoseconds: RFC3339 with Z, with offset, and the unparseable
  fallback path
• parse_headers: happy path, whitespace trimming, missing-colon errors, empty
  entries
• Cross-transport validation errors (syslog encoding on http transport and
  vice versa)