Reorganize the update mechanisms for logStartOffset, LocalStartOffset, and remoteLogStartOffset

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Fluss version

main (development)

### Please describe the bug 🐞

Reorganize the update mechanisms for logStartOffset, LocalStartOffset, and remoteLogStartOffset. The most critical part is in rebuildWriterState, where LogTablet#logStartOffset() is used instead of the current value of 0.

### Solution

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start at rebuildWriterState and inspect how LogTablet#logStartOffset(), logStartOffset, LocalStartOffset, and remoteLogStartOffset are currently read and updated. Trace each update path and verify that the reorganized mechanisms use consistent current values, especially where the issue identifies a default of 0; done means the offset behavior is covered by the existing relevant tests.

Written by the indexing model from the issue text.