Add Max button support for Predict Withdraw

**What is this about?**

The Max button was temporarily disabled for the Predict Withdraw flow in PR #27006 (fixes #26944) to unblock RC 7.68.0. The button was removed because pressing Max showed an amount significantly lower than the actual Predict balance, and fees alone did not explain the discrepancy.
This ticket tracks the work needed to re-enable the Max button with a correct implementation that fills the full Predict balance.

**Scenario**

1. Do a Predict Max Withdrawal

**Stakeholder review needed before the work gets merged**

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

Start by reviewing PR #27006 and issue #26944, then reproduce the Predict Max Withdrawal scenario described here. Trace the Predict Withdraw flow to determine why Max underestimates the balance; done means the button is re-enabled and fills the full Predict balance, with required automation tests passing.

Written by the indexing model from the issue text.