[Feature] Chained tables should allow set fallback rules for non-existent partitions.

### Search before asking

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

### Motivation

when using chain tables:
my partitions :
branch_snapshot: 20260101
branch_delta: 20260102、20260104

so i using: `select count(1) from table where dt=20260103` is 0
but in a scenario, It should return the most recent partition from the beginning of the timeline,
like above, should return data in 20260102
not 0.

### Solution

add a configuration, allow user to decide whether to fallback to the most recent partition from the beginning of the timeline

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start from the chained-table handling for a query such as `select count(1) from table where dt=20260103`, and trace how non-existent partitions are resolved. Review the configuration path for chained tables, then verify that an enabled fallback returns the most recent earlier partition while the existing behavior remains available.

Written by the indexing model from the issue text.