fix(rpc): narrow XDR error predicate, stop skipping on bare "unknown"

[makar52nn2016-jpg]

Closes #183 — fetchEventsSafe was silently skipping ledgers whenever the RPC error message contained the substring "unknown". That matched far too much: bare unknown error, getaddrinfo ENOTFOUND … unknown host, provider 5xx bodies. A transient network blip was indistinguishable from a genuinely unindexable ledger, and the cursor would advance past it without anything but a console.warn.

What changed

• New isXdrDecodeError(err) predicate covers the real XDR failure cases (SDK decoder messages containing "xdr", Soroban-specific "unknown scval type" / "unknown scaddress type" / "unknown soroban") and only those. Network blips, 5xx bodies, and bare "unknown" no longer match.
• fetchEventsSafe calls the predicate instead of the substring check in both the single-ledger branch (line ~209) and the multi-ledger bisection branch (line ~222).
• New ledgers_skipped_total Prometheus counter, labelled by network, increments on every XDR-skip. Skips are visible rather than only logged, so a sustained rate is alertable.

Acceptance criteria coverage

• Classification moves to a named predicate covering the real XDR cases and not bare "unknown"
• A transient Error("unknown error") propagates and is retried instead of skipping the ledger — fails before, passes after
• A genuine XDR error still bisects and skips; existing fetchEventsSafe tests stay green (20/20 pass)
• Every skipped ledger increments a metric, so skips are visible rather than only logged

Tests

src/__tests__/fetchEventsSafe.test.ts adds 13 new cases:

• isXdrDecodeError returns true for SDK decoder errors + Soroban unknown-type errors; false for bare "unknown", "unknown host", 5xx bodies, plain network errors, nullish inputs. Case-insensitive.
• fetchEventsSafe re-throws bare "unknown error" instead of skipping
• fetchEventsSafe re-throws "getaddrinfo ENOTFOUND … unknown host" instead of skipping
• fetchEventsSafe still skips a genuine XDR error on single-ledger range AND increments the new skip metric
• fetchEventsSafe still bisects genuine XDR errors on multi-ledger ranges
• fetchEventsSafe increments skipped-ledger metric for every XDR-skipped ledger

npx tsc --noEmit         # no errors
npx jest src/__tests__/fetchEventsSafe.test.ts  # 20/20 pass

The Integration tests CI job is red on main and predates this PR (per the issue body, open PRs #177 and #178 address it). If that job is red here too, it is not from these changes.

Bounty: Stellar Wave — Drips Wave — 150 points

[makar52nn2016-jpg]

Closing this PR.

Issue #183 is assigned to @JemimahEkong through the official Stellar Wave program workflow (application accepted on 2026-09-26, due 2026-09-30). I missed the application step and submitted this PR directly — that was incorrect. Maintainer's review queue should focus on the assigned contributor's work.

Apologies for the noise.