[upstream #4662] Fix #4651 default asset languagecode

[ayim]

Mirrored from vendurehq#4662 for the Overwatch review demo.

Original author: @kevmtt

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Description

Fixes vendurehq#4651.

Since v3.6, AssetService.createFromFileStream() fails with a NOT NULL constraint violation on asset_translation.languageCode when called without an explicit RequestContext. The root cause: the fallback used RequestContext.empty(), which constructs new Channel() with no defaultLanguageCode, leaving ctx.languageCode as undefined when createAssetInternal builds the default translation.

This PR introduces a new RequestContextService.createDefaultContext() method that builds a RequestContext based on the default Channel, guaranteeing a valid languageCode (falling back to ConfigService.defaultLanguageCode if the channel has none) and currencyCode. AssetService.createFromFileStream() now uses this helper instead of RequestContext.empty()` when no ctx is supplied, restoring the pre-v3.6 behavior.

Changes:

• packages/core/src/service/helpers/request-context/request-context.service.ts — add createDefaultContext() (admin API type, authorized, derived from the default channel).
• packages/core/src/service/services/asset.service.ts — inject RequestContextService and use createDefaultContext() as the fallback in createFromFileStream.
• packages/core/e2e/asset-create-from-file-stream.e2e-spec.ts — new e2e test reproducing the original issue by calling
  createFromFileStream without a RequestContext.
• docs/.../request-context-service.mdx — documents the new method.

Breaking changes

None. createDefaultContext() is additive, and the asset service change only affects the previously-broken path where no
RequestContext is passed (which threw a DB constraint error before).

Screenshots

N/A — backend-only change.

Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains a single feature
• I have checked my own PR

👍 Most of the time:

• I have added or updated test cases
• I have updated the README if needed