Count object separators when splitting data into chunks

[hiSandog]

splitData({ data: { a: 1, b: 2 }, maxChunkSize: 12 }) currently returns a single 13-character JSON object. Appending a property to a nonempty object removes two braces but adds a comma, so the combined length must subtract one character rather than three. The same undercount affects recursively split properties.

Correct both object-length calculations and add regression cases for ordinary properties, recursively split properties, and an object that exactly fits its limit.

Validation: all 19 tests in common/src/util/__tests__/split-data.test.ts pass with Bun 1.3.14, including two regression cases that fail before the fix. The two changed files also pass strict TypeScript checking, ESLint, and git diff --check.