getMempoolApiUrl has no signet or regtest case; signet/regtest nodes query the mainnet mempool API #4781

UrlUtils.getMempoolApiUrl has cases for a custom instance, Mutinynet, and testnet. There is no signet or regtest case, so on those networks every caller uses the mainnet API unless the user has set a custom mempool instance:

• FeeStore fee estimates
• TransactionsStore fee lookup (line 183) and broadcast (line 881)
• SweepStore UTXO lookup and transaction fetches

For sweeps, getUtxosFromAddress sends a tb1…/bcrt1… address to the mainnet API, the request fails, and the user sees the generic views.Wif.errorFetchingUtxos message with no hint about the cause.

Proposed fix:

• Signet: return https://mempool.space/signet/api when nodeInfo.isSigNet, checked after isMutinynet. Widen the parameter type to include isSigNet and isRegTest.
• Regtest: there is no public instance. Without a custom instance, fail with a specific message telling the user to set a custom mempool instance (new en.json string), instead of querying mainnet.

Caveat: a remote node on Mutinynet that reports signet without setting the mutinynet flag would go to the public signet API, which is the wrong chain. Today it goes to mainnet, which is also wrong, but the PR should check which backends set mutinynet.

Tests: getMempoolApiUrl for signet, regtest (with and without a custom instance), and Mutinynet reporting signet.

Reported by @ajaysehwal in #4758 (review). Also listed as out of scope in #4758.