feat: add agent preview debug mode - W-18417013

[shetzel]

What does this PR do?

Adds a new flag to the agent preview command to enable apex debug logs during agent conversation preview. When this flag is enabled, during each agent message send/retrieve that executes apex code there will be an apex debug log file written to the provided output directory.

What issues does this PR fix or reference?

@W-18417013@

[jshackell-sfdc]

See my suggestion.

[cristiand391]

QA notes:
setup:
afdx testing org with agent api user shared by Steve

✅ sf agent preview --connected-app-user agent-user -x creates proper trace flag for apex
✅ reuses existing trace flag (30min expiration window)
🟡 when passing --apex-debug, the final msg doesn't mention where the log file was written

🔴 If I answer No to save transcripts, no log is written:

[cristiand391]

QA update:

🟡 when passing --apex-debug, the final msg doesn't mention where the log file was written

✅ fixed

🔴 If I answer No to save transcripts, no log is written:

with --apex-debug it saves transcripts by default (no opt-out) so logs are always written

without --apex-debug saving transcripts can be skipped.