[ENH] Document magic constants throughout wal3/rust code

[rescrv]

Description of changes

This PR documents the remaining magic constants throughout the
wal3/rust-log-service codebase. The constants are generally OK, but
some probably deserve configuration.

This also catches a case where initial_insertion_epoch_us would only
move forward, and adds a descriptive error message to the case where
time moves backwards.

Test plan

CI, but it's mostly comments.

Documentation Changes

N/A

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[propel-code-bot]

Make Magic Constants Configurable and Document Their Use in wal3/rust-log-service

This PR audits all hardcoded magic constants throughout the wal3 and rust-log-service codebase, replacing several with configurable fields in the LogServerConfig struct. Previously hardcoded batch sizes, retry counts, timeouts, and similar values are now controlled via configuration options with sensible defaults. Some remaining magic constants are explicitly commented for future consideration, and a minor logic improvement ensures that time-related errors are handled more descriptively.

Key Changes:
• Replaced previously hardcoded magic constants (e.g., batch sizes, log keepalive durations, effectuate log transfer retries) in log-service with configuration fields, which now have serde defaults and descriptive documentation.
• Introduced new LogServerConfig fields: rollup_max_size, log_keep_alive, effectuate_log_transfer_batch_size, effectuate_log_transfer_retries, making numerous behaviors adjustable via configuration.
• Propagated these configurable values into relevant logic flows across logging, dirty log rolling, transfers, and compaction.
• Added TODOs/comments on remaining magic constants throughout wal3 (manifest.rs, writer.rs) for future configurability.
• Improved error handling for time moving backwards in LogService.

Affected Areas:
• rust/log-service/src/lib.rs
• rust/wal3/src/manifest.rs
• rust/wal3/src/writer.rs
• LogServerConfig definition and usage
• Compaction, log transfer, and dirty log handling logic

This summary was automatically generated by @propel-code-bot

[Sicheng-Pan]

Should leave a ticket to justify the magical constants or make them configurable