Bug: REST catalog auth cannot be configured via environment variables unless auth JSON strings are decoded

Empieza leyendo RestCatalog._create_session() y Config._from_environment_variables(), y luego reproduce los ejemplos documentados de variables de entorno de autenticación REST. Añade cobertura de regresión para la representación admitida de las variables de entorno y verifica que la inicialización del catálogo llegue al gestor de autenticación configurado sin tratar auth como una cadena inutilizable ni como un mapping con claves separadas por puntos.

Apache Iceberg version

None

Please describe the bug 🐞

Summary

RestCatalog._create_session() expects auth to be a dict. When catalog config comes from environment variables, values are strings, so auth is received as a string and auth initialization fails.

This blocks env-var-based configuration for pluggable REST auth (basic, oauth2, google, entra, custom) unless string JSON is explicitly decoded first.

Minimal repro

export PYICEBERG_CATALOG__REST__TYPE=rest
export PYICEBERG_CATALOG__REST__URI=http://localhost:8181
export PYICEBERG_CATALOG__REST__AUTH='{"type":"oauth2","oauth2":{"client_id":"id","client_secret":"secret","token_url":"https://auth.example/token"}}'

from pyiceberg.catalog import load_catalog
load_catalog("rest")

Actual (without this fix)

Expected: catalog initializes and uses the configured auth manager.

Actual: initialization fails because auth is treated as a string and .get(...) is called on it.

Suggested fix

In REST catalog session setup, if auth is a string, decode it as JSON before reading auth.type and type-specific config.

Add regression tests for both:

• PYICEBERG_CATALOG__<NAME>__AUTH (JSON string) initializes auth manager correctly.
• PYICEBERG_CATALOG__<NAME>__AUTH__... maps correctly into auth manager configuration.

Alternative fix (follows current env-var standard)

Support flattened auth properties from environment variables instead of requiring a JSON blob in ...__AUTH.

Example:

export PYICEBERG_CATALOG__REST__AUTH__TYPE=oauth2
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__CLIENT_ID=id
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__CLIENT_SECRET=secret
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__TOKEN_URL=https://auth.example/token

This aligns with existing flattened env-var configuration behavior and avoids JSON-in-env quoting/escaping issues.

Verification

Observed with current code path (Config._from_environment_variables):

export PYICEBERG_CATALOG__REST__AUTH__TYPE=oauth2
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__CLIENT_ID=id
export PYICEBERG_CATALOG__REST__AUTH__OAUTH2__CLIENT_SECRET=secret

Parsed result:

{'catalog': {'rest': {'auth.type': 'oauth2', 'auth.oauth2.client-id': 'id', 'auth.oauth2.client-secret': 'secret'}}}

This confirms flattened AUTH__... env vars are currently stored as dotted keys, not as a nested auth object consumed by RestCatalog._create_session().

Willingness to contribute

• I can contribute a fix for this bug independently
• I would be willing to contribute a fix for this bug with guidance from the Iceberg community
• I cannot contribute a fix for this bug at this time