[python] Propagate dependency read context in REST headers

[leaves12138]

Purpose

Port the dependency read context propagation introduced by #8823 and the custom catalog preservation follow-up from #8912 to PyPaimon.

Changes

• Add the optional X-Paimon-Read-Via REST header constant.
• Derive a dependency-scoped REST catalog context from the outermost table.
• Preserve the original source table across nested Blob View dependency reads.
• Preserve explicitly configured custom REST metastore identifiers.
• Reconstruct dependency catalogs through CatalogFactory with the full CatalogContext instead of forcing the built-in REST loader.
• Reuse existing header.* propagation so dependency table and data-token requests carry the context.
• Add focused tests for built-in REST, custom REST catalogs, external REST contexts, nested dependencies, non-REST catalogs, and Blob View loading.

The header is request context only and must not be treated as authorization proof.

Validation

• PYTHONPATH=. python3 -m unittest pypaimon.tests.catalog_environment_test pypaimon.tests.rest.api_test pypaimon.tests.rest.rest_token_file_io_test -v (28 tests passed)
• python3 -m compileall -q pypaimon/catalog/catalog_factory.py pypaimon/catalog/catalog_environment.py pypaimon/utils/blob_view_lookup.py pypaimon/tests/catalog_environment_test.py
• flake8 --config=paimon-python/dev/cfg.ini paimon-python/pypaimon/catalog/catalog_factory.py paimon-python/pypaimon/catalog/catalog_environment.py paimon-python/pypaimon/utils/blob_view_lookup.py paimon-python/pypaimon/tests/catalog_environment_test.py
• git diff --check