Fix inclusive Since comparison for historical tax rates

[rilla]

Context

While adding tests for historical rates in the FI regime implemented in #748, I realized that RateDef.Value() used Since.Before(date) (strictly less than) to match historical rates, so an invoice issued on the exact Since date would get the previous rate instead.

Existing regimes with historical rates (DE, CO, ES) define Since as the actual effective date (e.g., Since: 2017-01-01 meaning "from January 1st onwards"). The previous comparison silently shifted that by one day.

Changes

• Use !Since.After(date) to make Since an inclusive lower bound.
• Add unit tests for RateDef.Value() covering boundary, nil, and no-match cases

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Added thorough tests with at least 90% code coverage.
• Modified or created example GOBL documents to show my changes in use, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• Run go generate . to ensure that the Schemas and Regime data are up to date.
• Reviewed and fixed all linter warnings.
• Been obsessive with pointer nil checks to avoid panics.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[samlown]

Very nice catch. Clearly, we've never had a VAT rate chance since launching GOBL! Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.05%. Comparing base (926130f) to head (8c16a2e).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #750   +/-   ##
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  Coverage   93.05%   93.05%           
=======================================
  Files         333      333           
  Lines       17785    17785           
=======================================
  Hits        16549    16549           
  Misses        870      870           
  Partials      366      366           

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