descriptors.ts selectors ignore their `env` arg — silent wrong-descriptor risk if a second env is wired

Summary

Since #465, the three descriptor selectors in src/utils/descriptors.ts
(getAssetHubDescriptor / getBulletinDescriptor / getIndividualityDescriptor)
ignore their env parameter and unconditionally return the paseo-* descriptor.
This is safe today, but it is a latent footgun: the correctness of the selectors
depends on an implicit invariant enforced in a different file.

Background

@parity/product-sdk-descriptors@0.8.0 dropped the summit-* descriptor
subpaths (and summit was retired), so the selectors no longer branch per env —
they return the fixed paseo constants and discard the env arg:

export function getAssetHubDescriptor(_env: Env | undefined): AssetHubDescriptor {
    return paseo_asset_hub;
}
export function getBulletinDescriptor(_env: Env | undefined): BulletinDescriptor { … }
export function getIndividualityDescriptor(_env: Env | undefined): IndividualityDescriptor { … }

The risk

getConnection() (src/utils/connection.ts) builds the direct PAPI clients from
getAssetHubDescriptor(getChainConfig().env) (and the bulletin/individuality
equivalents). Because the env arg is discarded, "always return paseo" is only
correct while paseo-next-v2 is the sole env wired in CONFIGS.

If someone later wires a second env into CONFIGS (e.g. polkadot, which is
already present in ENV_IDS) without also editing descriptors.ts, direct
reads would decode that chain's state with the paseo descriptor — silently
wrong metadata, with no error raised. The only thing preventing this today is
getChainConfig() throwing for any env not in CONFIGS — an implicit safety net
one file away from the code that relies on it.

Proposed fix

Make the selectors fail loud instead of silently returning paseo for an
unexpected env: assert the requested env resolves to a paseo-family descriptor
(or that it is the wired default) and throw otherwise. ~3 lines; converts the
latent footgun into an immediate, obvious error the moment a second env is wired.
Restore real per-env selection here if/when another env ships dedicated
descriptors upstream.

Context

Surfaced by the #465 code review (altitude / latent-correctness finding).
Not a live bug — only paseo-next-v2 is wired today, and tsc + the
config.test.ts divergence guard both hold. Filed as hardening / tech-debt.

Start in src/utils/descriptors.ts by reading the three selectors and their paseo descriptor imports, then trace the env passed from src/utils/connection.ts. Ensure unexpected environments fail loudly while the wired paseo environment remains valid, and run tsc plus the config.test.ts divergence guard to verify the change.