store: skip chains in pool_size = 0 shards instead of panicking at startup (#6195)

[cargopete]

Summary

Fixes #6195. A node configured with pool_size = 0 for an unused shard (a
documented option from #3513) panics on startup:

WARN pool size for shard eth_chain is 0, ignoring this shard
thread 'main' panicked at node/src/store_builder.rs:203:
Creating the BlockStore works: InternalError("there is no pool for shard eth_chain")

store_builder correctly drops pool_size = 0 shards from the pool map, but
DieselBlockStore::new then iterates every chain — both configured chains
and all chains present in the database — and calls add_chain_store for each.
add_chain_store errors when a chain's shard has no pool, and that error is
.expect()ed in store_builder, killing startup. So pool_size = 0 is only
half-implemented: the pool is skipped, but the chains living in that shard are
not.

Fix

In both chain-enumeration loops in DieselBlockStore::new, skip chains whose
shard has no connection pool (logging a WARN) instead of calling
add_chain_store. This lets a node list all shards in config.toml but only
consume Postgres connections for the shards it actually uses, which is the
intent of pool_size = 0.

add_chain_store's contract is unchanged — it still errors when creating a
new chain in a shard with no pool, which remains a genuine misconfiguration.

Testing

• cargo fmt, cargo clippy --all-targets, and cargo check --release clean
  for graph-store-postgres.

I was not able to add an automated test: DieselBlockStore::new and
ConnectionPool require a live multi-shard Postgres, so exercising the
multi-shard startup path isn't possible in a unit test. The change is small and
verified by inspection, but I'd appreciate a maintainer (or @0x19dG87) confirming
startup against a real multi-shard config with pool_size = 0 set on an unused
shard.

[lutter]

Nice, just one small cleanup

[lutter]

It would be better to do this filtering when we construct existing_chains rather than every time we iterate over it - essentially, this logic treats a chain with pool_size == 0 as not exsiting/configured.