refactor(ibex): collapse payInvoice raw-fetch seam onto the SDK path

[islandbitcoin]

What the seam was

payInvoice in src/services/ibex/client.ts invoked the generated SDK through Ibex.authentication.withAuth(() => Ibex.ibex.payInvoiceV2(...)) with a .catch(httpErrorHandler), instead of the standard Ibex.payInvoiceV2(...).then(errorHandler) shape every sibling method uses. The seam existed because ibex-client 3.2.0's ApiError wrapper discarded the response body (lnflash/ibex-client#6): the raw FetchError (parsed body on .data) had to reach httpErrorHandler intact or insufficient-balance 400s were unclassifiable.

Why it's safe to collapse now

ibex-client 3.3.0 (bumped in #477) makes ApiError extract the body itself — httpCode / ibexResponse / ibexMessage — and errorHandler classifies that shape through the standard path. The wrapper's contract (withAuth(...).catch(_ => new ApiError(_)), resolving auth failures as AuthenticationError values) means every error class the seam handled now arrives at errorHandler with the same information:

• insufficient-balance 400s → InsufficientIbexBalance, ErrorLevel.Info, UUID-stripped client message, unstripped vendor detail preserved for logs/spans
• unclassified IBEX errors → generic IbexError (Warn) carrying the IBEX body detail exactly once (no duplication with ApiError's own embedded copy), capped at MAX_IBEX_MESSAGE_LENGTH for oversized (e.g. Cloudflare HTML) bodies
• auth failures → IbexError at ErrorLevel.Critical
• network-level failures / non-Error throwables → generic IbexError without a status

What happened to httpErrorHandler

Removed. After the collapse it had zero production callers (ibexFetch/ibexPost build their own IbexError inline and never routed through it), and a fully-dead defense-in-depth handler rots. Its dedicated describe block is removed with it; every behavioral pin it carried was ported onto the errorHandler path (see below), so no assertion coverage was lost — only the duplicate route to it.

Tests

• test/flash/unit/services/ibex/errors.spec.ts — the "ibex-client 3.3.0 integration" describe now pins the full collapsed-path behavior with real ApiError construction (the exact shape the wrapper resolves with): JSON and non-JSON insufficient-balance bodies, casing preservation, case-insensitive classification, payment-already-prepared, unclassified detail carried exactly once with status + level, bounded message on oversized bodies, network-level failure without status, non-Error throwable tolerance, status preservation on body-less errors.
• test/flash/unit/services/ibex/pay-invoice.spec.ts — mock now mirrors ibex-client 3.3.0's wrapper contract (unwrap .data, resolve AuthenticationError, resolve thrown errors as new ApiError(err)) instead of stubbing withAuth; adds a payInvoice-level auth-failure test pinning ErrorLevel.Critical.
• Targeted suite: 7 suites, 70 passed. Full yarn test:unit: 181 suites, 1585 passed / 3 skipped. yarn tsc-check clean; eslint clean on all changed files.

Fixes #478

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