fix(react): useRxDocument initial loading state is true when collection and key are provided

[dikshit-n]

Summary

Fixes useRxDocument starting with loading: false on first render even when a valid collection and primaryKey are provided, causing a false "not found" flash before the subscription resolves.

Problem

In useRxDocument, the loading state was initialized to false:

const [loading, setLoading] = useState(false);

But setLoading(true) only runs inside useEffect. The first committed React paint always shows loading: false and result: null, which is indistinguishable from "document not found".

This was reported in #8965.

Solution

Initialize loading to true when collection is not null and primaryKey is not undefined:

const [loading, setLoading] = useState(collection !== null && primaryKey !== undefined);

This matches the established pattern in useRxQuery and useLiveRxQuery, both of which start with loading: true when valid inputs are provided.

Changes Made

• src/plugins/react/hooks/use-rx-document.ts: Initialize loading state based on whether valid inputs are provided
• test/react/react-hooks.test.tsx: Add 5 tests for useRxDocument covering loading state and result behavior
• orga/changelog/fix-use-rx-document-initial-loading-state.md: Changelog entry

Testing

• Unit tests added: 5 new test cases in test/react/react-hooks.test.tsx
• Tests added following existing patterns (renderHook, waitFor, assert.strictEqual)

Checklist

• Tests added following existing repo patterns
• Code passes local test suite (CI will verify)
• Lint passes (CI will verify)
• Code follows repo style
• Documentation updated (changelog entry added per CLAUDE.md)
• No console.log or debug code left
• No unrelated changes