test: cover self-freeze confirmation flow

[mayborn005]

Summary

The self-freeze feature for portfolio holdings is already present on dev; this PR adds focused regression coverage for the user-facing confirmation and balance guard behavior so the implementation remains protected.

Validation

• Covers confirmation submission with a requested amount
• Covers rejection of amounts above the liquid balance
• git diff --check passes

Closes #936

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[mayborn005]

@accesslayerorg Please review this focused regression coverage for #936. The freeze implementation is already on dev; this PR adds tests for the confirmation step and available-balance guard.

Closes #936