api: make pool-config and worker-selection methods required on SubstrateAdapter #884

Problem

In crates/libs/api/src/controllers/substrate/mod.rs, four methods on SubstrateAdapter still carry default implementations that return SubstrateError::Unsupported:

• list_pool_configs (mod.rs:1609)
• upsert_pool_config (mod.rs:1616)
• disable_pool_config (mod.rs:1626)
• select_ready_worker_node_api_target (mod.rs:1648 — already implemented by both DirectSubstrate and KubernetesSubstrate and forwarded by Substrate, making the trait default dead code)

crates/libs/api/src/controllers/substrate/AGENTS.md states that SubstrateAdapter operations must be required methods so the compiler enforces an explicit implementation on each substrate and a forwarding arm in impl SubstrateAdapter for Substrate, and notes that the Unsupported trait defaults on list_pool_configs, upsert_pool_config, and disable_pool_config are debt.

Why it matters

Default Unsupported bodies on SubstrateAdapter weaken compile-time exhaustiveness across substrates: a missing arm in impl SubstrateAdapter for Substrate or on a substrate implementation silently returns 501 Not Implemented instead of failing to compile.

Proposed approach

1. Remove the default bodies from list_pool_configs, upsert_pool_config, disable_pool_config, and select_ready_worker_node_api_target on trait SubstrateAdapter in crates/libs/api/src/controllers/substrate/mod.rs.
2. Add explicit list_pool_configs, upsert_pool_config, and disable_pool_config implementations to impl SubstrateAdapter for DirectSubstrate in crates/libs/api/src/controllers/substrate/direct.rs returning the same SubstrateError::Unsupported error.
3. Update crates/libs/api/src/controllers/substrate/AGENTS.md to remove the resolved trait-default debt reference.

Severity

Low (compile-time contract hardening; zero runtime behavior change).