Fix: return to Agents list when tapping back on a newly created agent DM

[MelvinBot]

Explanation of Change

On narrow (native) layouts, creating an agent revealed the new DM as a fullscreen report and then dismissed the entire Settings → Agents → New agent modal stack (revealRouteBeforeDismissingModal). That left the Agents list off the back stack, so tapping back on the agent chat fell through to the LHN instead of returning to Agents.

This passes a backTo param pointing at the Agents list (ROUTES.SETTINGS_AGENTS) when revealing the DM, so the report header's back button (ReportHeader.onBackButtonPress → Navigation.goBack(backTo)) returns to the Agents list as expected.

Wide layouts are unchanged: they already open the DM inside the RHP (AGENT_REPORT with forceReplace) and correctly return to Agents. Only the narrow-layout branch is affected. Follow-up to #95631.

Fixed Issues

$ #97832
PROPOSAL:

Tests

// TODO: The human co-author must confirm the tests below before marking this PR as "ready for review".

Suggested steps (on a narrow layout — native app or a narrow browser window):

1. Go to Account > Agents.
2. Tap New agent and create a new agent.
3. You land on the new agent DM chat. Tap the back button in the chat header.
4. Verify you return to the Agents list (not the LHN / report list).
5. On a wide layout, repeat and verify closing the agent DM still returns to the Agents list (no regression).

• Verify that no errors appear in the JS console

Offline tests

Same as tests — the agent create + navigation flow is fully optimistic and works online or offline.

QA Steps

// TODO: The human co-author must confirm the QA steps before marking this PR as "ready for review". Same as tests.

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
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• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari