[Feature] [python] Align ExternalPathProvider with Java multi-strategy support

[zhoulii]

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• I searched in the issues and found nothing similar.

Motivation

The Java side supports multiple path selection strategies via ExternalPathProvider interface with three implementations. The Python side only had hard-coded round-robin logic, meaning:

1. Tables configured with entropy-inject strategy would produce different directory layouts between Java and Python writers, breaking cross-engine reads
2. Tables configured with weight-robin strategy could not be written by pypaimon at all

Solution

aligns pypaimon's ExternalPathProvider with the Java implementation by supporting all external path selection strategies: round-robin, specific-fs, entropy-inject, and weight-robin.

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!