Review node ID handling in IdGenerator

### Is your feature request related to a problem? Please describe.

Following up on discussions in #2131, the handling of node IDs in the Id Generator SPI and impl. needs to be reviewed.

Specifically:
* Making `IdGeneratorSource` immutable. Cf. https://github.com/apache/polaris/pull/2131#discussion_r2215576536
* Keeping Node IDs internal to the Snowflake ID implementation. Cf. https://github.com/apache/polaris/pull/2131#discussion_r2216777142

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by locating the IdGenerator SPI, IdGeneratorSource, and Snowflake ID implementation, then read the discussions linked from issue #2131. Determine the intended immutability boundary and whether node IDs remain internal to Snowflake ID handling; done means those design questions are resolved and the affected interfaces and implementation are consistently updated.

Written by the indexing model from the issue text.