Migrate to native flag-engine v10 APIs and remove vendored models

[gagantrivedi]

Context

This is a follow-up to #5991 and the engine v10 compatibility work.

Phase 1 (completed in #6653) upgraded flagsmith-flag-engine to v10 and vendored Pydantic models from the fix/missing-export branch to maintain backwards compatibility with the existing codebase.

This issue tracks Phase 2: fully migrating to flag-engine v10's native APIs and removing the vendored code.

Goals

• Remove vendored models from api/util/engine_models/

• Remove dependency on legacy Pydantic models

Additional work

• Update flagsmith-schemas (in flagsmith-common) to use v10's ContextValue type with correct ordering/constraints, or accept the schema changes
• Remove vendored code in api/util/engine_models/
• Update any mappers in api/util/mappers/engine.py that depend on vendored models

Related

• Parent issue: Context Values: Bump flagsmith-flag-engine to ^10.0.3. #5991
• Phase 1 PR: feat: upgrade flagsmith-flag-engine to v10 #6653

[gagantrivedi]

More context (that may or may not be accurate):

[-]- [ ] Add Django to Evaluation Context mappers. [/-]
[-]- [ ] Migrate to `get_evaluation_result`: https://github.com/Flagsmith/flagsmith/blob/fafc41a5c8324c50667b41bc6fb67f4706b619f0/api/environments/dynamodb/wrappers/identity_wrapper.py#L177-L201[/-]
[-]- [ ] Migrate to `get_evaluation_result`: https://github.com/Flagsmith/flagsmith/blob/fafc41a5c8324c50667b41bc6fb67f4706b619f0/api/integrations/webhook/serializers.py#L34-L44[/-]
[-]- [ ] Migrate to `get_evaluation_result`: https://github.com/Flagsmith/flagsmith/blob/fafc41a5c8324c50667b41bc6fb67f4706b619f0/api/environments/identities/models.py#L143-L188[/-]
[-]- [ ] Vendor in trait update logic from the legacy engine: https://github.com/Flagsmith/flagsmith/blob/fafc41a5c8324c50667b41bc6fb67f4706b619f0/api/edge_api/identities/views.py#L184-L201[/-]
[-]- [ ] Make sure the tests pass.