fix(core): resolve sparse array and rolling size accumulator regressions in event chunking (#1309)

[Mohitingale13]

What does this PR change?

Fixes two batching regressions in chunk() that can stall event flushing and fragment event batches.

Changes

• Prevents sparse array holes when an oversized event is the first item in a batch.
• Resets the rolling payload-size accumulator when starting a new batch.
• Adds regression coverage for oversized events, consecutive oversized events, oversized events between normal events, and exact maxKB boundaries.
• Adds an integration regression test covering the SegmentDestination flush path when encountering an oversized initial event.

Verification

• Targeted tests: 70 passed across util.test.ts and SegmentDestination.test.ts
• Full test suite: 70 test suites / 510 tests passed
• TypeScript typecheck: 0 errors
• ESLint: 0 errors
• Full build: passed

Closes #1309