[config] new testnet env vars BNB_TESTNET_WS_URL / BNB_TESTNET_HTTP_URL violate CHARON_* namespace convention

[obchain]

PR: #51 (feat/23-testnet-config)
File: config/testnet.toml (new), .env.example
Refs #51

Problem

The PR description names the two new environment variables as BNB_TESTNET_WS_URL and BNB_TESTNET_HTTP_URL. These do not follow the CHARON_* prefix convention required for all env vars in this project.

Prior findings on this pattern:

• PR feat(executor): private-RPC transaction submitter #44 finding 7: BSC_PRIVATE_RPC_URL must be CHARON_BSC_PRIVATE_RPC_URL
• PR feat(cli): wire scanner → router → builder → simulator pipeline #42 finding 5: BOT_SIGNER_KEY must be CHARON_SIGNER_KEY

The existing mainnet vars BNB_WS_URL / BNB_HTTP_URL in .env.example are also non-compliant (pre-existing gap). Adding two more in the same style compounds the technical debt.

Impact

In Docker Compose / Kubernetes environments with multiple service containers, bare BNB_TESTNET_* names risk collision with other services. The CHARON_ prefix is the namespace boundary that prevents this.

Fix

Rename the new vars to:

• CHARON_BNB_TESTNET_WS_URL
• CHARON_BNB_TESTNET_HTTP_URL

Update config/testnet.toml references accordingly. As part of this PR, also rename the pre-existing mainnet vars to CHARON_BNB_WS_URL and CHARON_BNB_HTTP_URL in .env.example and config/default.toml — all three files change atomically so operators have a single consistent namespace from this PR forward.

[obchain]

Fixed on feat/23-testnet-config (d0c5278): testnet env vars renamed to CHARON_BNB_TESTNET_*.