Extract FormatParser trait, simplify pipeline hot loop

[strawgate]

Summary

• Introduce FormatParser trait in logfwd-core::format with three implementations:
  • JsonParser — passes through newline-delimited JSON, carries partial lines
  • RawParser — wraps lines as {"_raw":"<escaped>"}
  • CriParser — parses CRI container log format, reassembles partials
• Simplify pipeline_v2.rs::InputState from 4 format-related fields to one Box<dyn FormatParser>
• Hot loop reduced from a 35-line format match to: input.parser.process(&bytes, &mut input.json_buf)
• New formats (logfmt, syslog — already in the config enum) just need to implement the trait

Before (pipeline_v2.rs hot loop)

match input.format {
    Format::Cri => { cri::process_cri_to_buf(...) }
    Format::Json | Format::Auto => { accumulate_json_lines(...) }
    Format::Raw => { accumulate_raw_lines(...) }
    _ => { accumulate_json_lines(...) }
}
// + separate reassembler.reset() and partial_line.clear() on rotation

After

let n = input.parser.process(&bytes, &mut input.json_buf);
// rotation: input.parser.reset();

Test plan

• cargo test --workspace — 93 tests pass (91 original + 6 new format tests - 4 moved)
• cargo clippy --workspace -- -D warnings — clean
• cargo fmt --check — clean
• No behavioral changes — format parsing logic identical, just moved behind trait

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