Add date-generation rule to SwapRateHelper

[snigdhachoppac]

This adds an optional DateGeneration::Rule parameter to SwapRateHelper, addressing the remaining part of #1393.

The new parameter:

• is available on all three SwapRateHelper constructors
• defaults to DateGeneration::Backward, preserving existing behavior
• is passed to MakeVanillaSwap::withRule
• applies to both the fixed and floating schedules

Regression coverage verifies the default backward rule and an explicitly supplied forward rule for both the tenor-based and explicit-date constructors.

Fixes #1393

Tests

• Focused SwapRateHelper date-generation regression test
• PiecewiseYieldCurveTests: 34/34 passed
• Full optimized test suite: 1367/1367 test cases and 12413/12413 assertions passed

[CLAassistant]

All committers have signed the CLA.

[boring-cyborg]

Thanks for opening this pull request! It might take a while before we look at it, so don't worry if there seems to be no feedback. We'll get to it.

[lballabio]

It would be nice to use the IMM rule, as asked in the issue, and check the resulting swap dates as well as the rule.

[lballabio]

@snigdhachoppac any chance you can work on this?