Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@libs/sdk/src/backend/index.ts`:
- Around line 100-102: The Backend interface and BackendAPI7 are mismatched:
tests call backend.supportValidate() but the interface only defines
supportStreamRoute; add an optional supportValidate?: () => Promise<boolean> to
the Backend interface in libs/sdk/src/backend/index.ts and update BackendAPI7 to
implement supportValidate (either by delegating to supportStreamRoute or
returning the correct boolean) so backend.supportValidate() exists at runtime.

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@libs/backend-api7/e2e/validate.e2e-spec.ts`:
- Around line 139-140: The assertion is order-dependent by checking
result.errors[0].resource_type; replace it with a membership-style check against
the result.errors array (e.g., assert that result.errors contains at least one
element whose resource_type === 'routes') so the test does not rely on error
ordering; locate the assertions referencing result.errors in the
validate.e2e-spec.ts test and update them to use an existence/contains check
(e.g., Array.prototype.some or an equivalent matcher) instead of indexing [0].
- Line 39: The test uses Jest's .rejects on an Observable (backend.validate([]))
which requires a Promise; change the assertion to wrap the Observable with
lastValueFrom (from 'rxjs') before applying .rejects, i.e. use
lastValueFrom(backend.validate([])) in the expect call; ensure any existing
lastValueFrom imports in the file are used or add the import if missing and keep
the rest of the assertion message ('not supported') unchanged.