refactor(httpapi): describe bodyless global upgrade payload

[kitlangton]

Summary

• Declare POST /global/upgrade as accepting either GlobalUpgradeInput or HttpApiSchema.NoContent.
• Keep the existing raw handler and runtime behavior unchanged.
• Add focused tests for its existing bodyless-upgrade and malformed-JSON behavior.

Context

/global/upgrade already treats an absent request body as {} and uses that to select the latest version. Its HttpApi declaration described only the object body, so this aligns the declared contract with existing behavior.

This is an internal contract correction and regression-coverage improvement, not a user-facing runtime bug fix: bodyless requests already succeeded before this change. HttpApiSchema.NoContent is the intended HttpApi representation for that supported bodyless request shape.

Verification

• bun prettier --write src/server/routes/instance/httpapi/groups/global.ts test/server/httpapi-global.test.ts
• bun oxlint packages/opencode/src/server/routes/instance/httpapi/groups/global.ts packages/opencode/test/server/httpapi-global.test.ts
• bun typecheck from packages/opencode
• bun run test -- test/server/httpapi-global.test.ts from packages/opencode
• ./packages/sdk/js/script/build.ts (no generated SDK changes)
• Pre-push hook: bun turbo typecheck passed across the workspace