Handle equal-return edge case in CLA max_sharpe

[Whning0513]

Summary

• treat None lambdas consistently in CLA case-b comparisons
• let max_sharpe() fall back to the same solution as min_volatility() when expected returns are equal
• add a regression test covering equal expected returns

Closes #738

[Whning0513]

Closing this as a duplicate of the earlier #740. Both apply the same _infnone guard and add the same equal-return regression case; keeping a second implementation open does not add a distinct fix.