fix(payment): skip the Solana balance check when no Tatum API key is configured

[Danswar]

Why

The hourly PaymentBalanceService Solana balance check on dev fails on every run:

SolanaJSONRPCError: failed to get balance for DwS2f...: Method 'getBalance' is
available for paid plans only.

The error text reads like a subscription-tier problem, but the actual root cause is that TATUM_API_KEY is not set on dev at all:

• The Cardano guard from fix(payin): skip Cardano pay-in check when no Tatum API key is configured #3878 reads the same env var (cardanoTatumApiKey = TATUM_API_KEY), and its warn — "Cardano has no Tatum API key configured — skipping pay-in check" — fires on dev after every boot and never on prod.
• A keyless request to solana-mainnet.gateway.tatum.io reproduces the error byte-identically: Tatum's anonymous tier serves basic JSON-RPC (getSlot works) but gates getBalance.

So dev's Solana client has been calling Tatum anonymously, and the only method that minds is the one the hourly balance check needs. #3878 scoped Solana out of the unconfigured-guard on the grounds that it made no unprompted Tatum calls — the hourly PaymentBalanceService check (via the finance-log cron) is exactly such a call, so that assumption no longer holds.

What

Extends the #3878 pattern to this caller:

• BlockchainClient base: get isConfigured() (default true) — same semantics as CardanoClient's existing getter.
• SolanaClient override: Tatum API key present.
• PaymentBalanceService.getPaymentBalances: the existing not-configured warn-once-and-skip branch now also covers a client that exists but is unconfigured.

Prod (key set) is unchanged, and a key that is set but rejected still errors — a real misconfiguration stays visible. Tron is deliberately not gated: its keyless balance calls show no failures on dev.

Companion: DFXServer/server#730 provisions a Grafana alert on the not-configured warn scoped to prod, so a TATUM_API_KEY lost on prod (where the guard would otherwise reduce it to one boot-time warn) still pages.

Replaces the PaymentBalanceService half of #4200 (suppressed the provider error string in shared code) and #4202 (RPC-provider swap built on the wrong "paid plan lacks the method" premise) — both closed.

Test plan

• New payment-balance.service.spec.ts: unconfigured client → skipped, single warn, no error; configured client → balances fetched; configured client whose call rejects → still throws / logs ERROR (locks the set-but-rejected contract). 3/3 passing
• Full local suite green (pre-push hook)
• Lint / tsc --noEmit / prettier clean
• After dev deploy: hourly Solana error stops; single Blockchain client not configured for Solana warn per boot instead