fix(image_variant): fix nested variant response deserialization

[Monark-Arkmon]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

HAND-PATCH: needs reseal
This fixes generated files (model.go, resource.go, schema.go) directly, because the root cause is inside Stainless's own generator, not in our OpenAPI spec or Stainless config. Confirmed via a real preview build (config fix tested, verified via actual generated artifact to have zero effect). See IMAGES-1782 / APIX-1226 for the full history of attempted upstream fixes.

The API returns image variant responses with data nested under result.variant, but Create/Read/Update/ImportState were deserializing against a flat envelope, so options.fit/metadata came back null after any Update unless masked by a no_refresh tag on the affected fields.

Switches to ImageVariantWriteResultEnvelope, which correctly parses the nested result.variant shape, and removes the now-redundant computed variant field from the resource schema/model.

Acceptance test run results

• I have added or updated acceptance tests for my changes
• I have run acceptance tests for my changes and included the results below

Steps to run acceptance tests

TF_ACC=1 CLOUDFLARE_API_TOKEN=<token> CLOUDFLARE_ACCOUNT_ID=<account_id> \
  go test ./internal/services/image_variant/... -run "TestAccCloudflareImageVariant" -v -timeout 5m

Test output

=== RUN   TestAccCloudflareImageVariantDataSource_Basic
--- PASS: TestAccCloudflareImageVariantDataSource_Basic (8.44s)
=== RUN   TestAccCloudflareImageVariant_Basic
--- PASS: TestAccCloudflareImageVariant_Basic (4.71s)
--- PASS: TestAccCloudflareImageVariant_Import (6.44s)
--- PASS: TestAccCloudflareImageVariant_Update (7.90s)
PASS

Additional context & links

Supersedes #6918 (same underlying fix, redone on top of current main, with acceptance tests actually run and passing against a real account).
Ref IMAGES-1782 / APIX-1226.