fix: use the error's oclif exit code when --json is enabled

[mikaelkaron]

What does this PR do?

With --json enabled, Command.catch sets the exit code from err.exitCode:

process.exitCode = process.exitCode ?? err.exitCode ?? 1

CLIError and ExitError don't set exitCode; they carry their code in err.oclif.exit. So under --json, every failure exits with 1:

this.error('Missing argument', {exit: 2}) // exit 2, but exit 1 with --json
this.exit(3)                              // exit 3, but exit 1 with --json

Without --json, handle() already exits with err.oclif.exit. A script that checks $? gets a different code depending on whether it asked for JSON.

This PR makes catch prefer err.oclif.exit, then err.exitCode, then 1:

process.exitCode = process.exitCode ?? err.oclif?.exit ?? err.exitCode ?? 1

CommandError gains the optional oclif metadata (Partial<OclifError>), so the type matches what catch receives.

Testing

New tests in test/command/command.test.ts, under --json exit code:

• this.error(..., {exit: 2}) exits 2;
• this.exit(3) exits 3;
• an error with exitCode: 4 and no oclif.exit still exits 4;
• a plain Error still exits 1.

The first two fail on main (exit 1) and pass with this change. yarn test passes (836), and so do yarn lint and test:circular-deps.

[salesforce-cla]

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