feat: add DiscountEligibilityStep pipeline step (#2545)

* feat: add DiscountEligibilityStep pipeline step

Adds enterprise/filters/discounts.py with a DiscountEligibilityStep
PipelineStep implementation that marks enterprise learners as ineligible
for LMS-controlled course discounts, using the new
DiscountEligibilityCheckRequested openedx-filter.

ENT-11564

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: log pipeline step

* chore: rename step to reference Enterprise

* docs: add function type annotations

* fix: add filter to ENTERPRISE_FILTERS_CONFIG

* chore: pylint disable arguments-differ

* chore: fix unused test variable lint error

* chore: fix pylint spacing

* chore: isort fixes

* chore: bump version

* feat: raise DiscountIneligible exception

* fix: remove is_eligible

* test: remove is_eligible

* test: remove unneeded mocks

* chore: update openedx-filters to 3.7.0

* chore: isort fix

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Marlon Keating <mkeating@2u.com>