SDK's Network.connect has no headless/private-key signing option

### Summary

`@hashgraph/asset-tokenization-sdk`'s `ConnectRequest.wallet` field only
accepts `SupportedWallets.{METAMASK, HWALLETCONNECT, DFNS, FIREBLOCKS, AWSKMS}`
(confirmed from the installed package's own `ConnectRequest.d.ts`, v8.0.0).
There doesn't appear to be a way to drive the SDK from a plain Node.js
script using a raw Hedera operator private key — every option needs
either a browser wallet extension, an interactive WalletConnect pairing,
or an enterprise custodial KMS account already set up.

### Why this is surprising

`packages/ats/sdk/__tests__/config.ts` in this repo constructs an
`Account` directly from a raw ECDSA `PrivateKey`:

```ts
export const CLIENT_PRIVATE_KEY_ECDSA = new PrivateKey({
key: process.env.CLIENT_PRIVATE_KEY_ECDSA_1 ?? "",
type: "ECDSA",
});
// ...
export const CLIENT_ACCOUNT_ECDSA: Account = new Account({
id: CLIENT_ACCOUNT_ID_ECDSA,
evmAddress: CLIENT_EVM_ADDRESS_ECDSA_1,
privateKey: CLIENT_PRIVATE_KEY_ECDSA,
publicKey: CLIENT_PUBLIC_KEY_ECDSA,
});
```

This pattern is clearly used internally by the SDK's own integration test
fixtures, but it isn't exposed as a supported `Network.connect` path for
consumers. For anyone building a deploy script, indexer, keeper bot, or
CI job that needs to issue/manage securities without a browser in the
loop, this is a dead end as far as the public SDK surface goes.

### Suggested fix (either would help)

1. **Document it explicitly** — a line in the SDK README's "Connect an
account to the SDK" section stating that headless/backend usage isn't
currently supported by the public API would have saved real
integration time.
2. **Or expose the raw-key path** — since the underlying capability
clearly exists (the test fixtures use it), surfacing it as a real
`SupportedWallets` option (e.g. `SupportedWallets.PRIVATE_KEY` /
`SupportedWallets.OPERATOR_KEY`) would let backend integrators use the
SDK the way its own tests already do.

### What we did instead

Called the ATS diamond contracts directly via
`@hashgraph/asset-tokenization-contracts`'s shipped
`deployEquityFromFactory` helper and generated typechain types, driven by
a plain `ethers.Wallet`. Worked well, but required reading source to
discover — the SDK's advertised path didn't get us there. Full writeup:
https://github.com/fabian4819/Stratus-Finance/blob/main/docs/phase-0-findings.md

### Environment

- `@hashgraph/asset-tokenization-sdk` v8.0.0
- `@hashgraph/asset-tokenization-contracts` v8.0.0
- Node.js v22

Start at the public Network.connect entry point and the installed ConnectRequest.d.ts, then compare the available wallet options with packages/ats/sdk/__tests__/config.ts. Review the SDK README's “Connect an account to the SDK” section and the linked findings for context. Done means the chosen scope is resolved: either headless support is exposed consistently or its lack is explicitly documented.

Written by the indexing model from the issue text.