[codex] Normalize generated load query variables

[zth]

Summary

• Normalize variables inside the public RescriptRelayReact.loadQuery wrapper so generated *.load helpers pass {} instead of undefined for no-variable operations.
• Add provided-variable coverage for no-variable query paths, including generated raw load, useLoader, fetch, fetchPromised, and retain.
• Add no-variable provided-variable coverage for useMutation and subscriptions.

Root Cause

Generated query load helpers call RescriptRelayReact.loadQuery directly with variables->Internal.convertVariables. For operations whose user variables type is unit, that conversion can produce undefined; Relay then crashes while resolving provided variables from that value.

Validation

• yarn build:test
• yarn build
• yarn test --runTestsByPath __tests__/Test_providedVariables-tests.js __tests__/Test_mutation-tests.js __tests__/Test_subscription-tests.js --runInBand
• yarn test:ci

[zth]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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