fix(cli): accept --format after a subcommand

[sanjay3290]

Problem

Fixes #6391.

--format is declared on the top-level Cli struct without global = true, so clap only accepts it before the subcommand:

$ buzz --format compact messages get --channel <uuid>   # works
$ buzz messages get --format compact --channel <uuid>   # error: unexpected argument '--format'

The second form is what people actually type, and it is how every other CLI treats an output-format flag.

Change

One line: add global = true to the format arg. OutputFormat also gains Debug and PartialEq so the tests can assert on it.

Tests

Three tests in the existing mod tests, all driving the real Cli parser via Cli::try_parse_from, not a synthetic command:

• format_after_subcommand_parses — the previously broken form; fails on main with an unexpected-argument error.
• format_before_subcommand_parses — the form that already worked, so the fix does not trade one position for the other.
• format_defaults_to_json — the default survives.

Deletion probe: removing global = true makes format_after_subcommand_parses fail.

Verification

cargo fmt --all -- --check                              # clean
cargo clippy --workspace --all-targets -- -D warnings   # clean
cargo test -p buzz-cli                                  # 366 passed, 0 failed