feat(provider): add NVIDIA endpoints origin header

[nv-kasikritc]

Issue for this PR

N/A

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds X-BILLING-INVOKE-ORIGIN: OpenCode to NVIDIA provider requests so NVIDIA can identify requests originating from opencode. The header is added through the same provider-level header path used for existing NVIDIA/OpenRouter attribution headers.

The default billing-origin header is applied for hosted NVIDIA and custom/local NVIDIA NIM baseURL configurations so app-origin attribution is consistent across NVIDIA provider requests. Explicitly configured X-BILLING-INVOKE-ORIGIN values are preserved.

How did you verify your code works?

• Ran git diff --check
• Ran bun typecheck from packages/core
• Ran bun typecheck from packages/opencode
• Ran bun test test/v2/plugin/provider-nvidia.test.ts from packages/core (4 pass)
• Ran bun test test/provider/provider.test.ts --test-name-pattern nvidia from packages/opencode (3 pass, 78 filtered)
• Pre-push hook ran bun turbo typecheck successfully (14 tasks)
• Earlier full bun test from packages/core: 333 pass, 1 unrelated failure in cross-spawn-spawner.test.ts caused by macOS temp path canonicalization (/var/... expected, /private/var/... received)
• Earlier full bun test from packages/opencode: 2298 pass, 282 fail, 49 errors; failures were outside the NVIDIA provider path and dominated by FSEvents, port binding, and timeout issues

Screenshots / recordings

N/A. This is not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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