fix(sdk-provider-solana): preserve Solana RPC error details on TransactionError cause

[chybisov]

Summary

• Introduce SolanaTransactionDetailsError, a minimal Error subclass exposing err and logs as own readonly properties — exported from the package root so consumers can instanceof and type-narrow.
• Apply it uniformly at all three Solana throw sites that previously stringified the structured RPC result into a message and discarded the original payload.
• Consumers can now read error.cause.err and error.cause.logs directly, one level deep — without re-simulating the transaction.

Credit

Big thanks to @samsamtrum for surfacing the issue in #385 and shipping the first-pass fix in #386 — this PR builds directly on that diagnosis. It supersedes #386 by flattening the cause shape and extending the same fix to two sibling sites.

Why

Closes #385.
Supersedes #386 — please close it after this lands.

The Solana provider has three RPC-driven failure paths. Each one used to stringify the structured Solana result into a message and lose the rest:

Site	Before	After
SolanaStandardWaitForTransactionTask preflight simulation	cause was new Error(msg, { cause: { err, logs } }) (nested two levels)	cause is SolanaTransactionDetailsError with .err and .logs
SolanaStandardWaitForTransactionTask post-send signatureResult.err	no cause at all	cause is SolanaTransactionDetailsError with .err
SolanaJitoWaitForTransactionTask bundle failedResult.err	no cause, no bigint-safe JSON.stringify (latent bug if err contains bigints)	cause is SolanaTransactionDetailsError with .err; bigint payloads now serialize safely

Bigint-safe JSON.stringify is now a single helper (safeStringifyBigInt), replacing three near-duplicate inline copies.

Public surface

• TransactionError.code and message prefixes (Transaction simulation failed: …, Transaction failed: …) are preserved verbatim.
• error.cause is strictly more informative; was a wrapper Error or undefined before.
• SolanaTransactionDetailsError is exported from @lifi/sdk-provider-solana for typed consumer access:

import { SolanaTransactionDetailsError } from '@lifi/sdk-provider-solana'

try {
  await sdk.executeRoute(route)
} catch (error) {
  if (error.cause instanceof SolanaTransactionDetailsError) {
    console.log(error.cause.err)   // structured Solana error
    console.log(error.cause.logs)  // program execution logs (preflight only)
  }
}

Test plan

• pnpm check (biome) — clean
• pnpm check:types — clean (all 6 packages)
• pnpm check:circular-deps — clean
• pnpm knip:check — clean
• pnpm --filter @lifi/sdk-provider-solana test:unit — 49 passed (16 new)
• pnpm test:unit (monorepo) — all packages green
• pnpm build (monorepo) — all packages built; SolanaTransactionDetailsError appears in dist/esm/index.d.ts

New unit tests:

• solanaErrorCause.unit.spec.ts — safeStringifyBigInt + SolanaTransactionDetailsError behaviour
• SolanaStandardWaitForTransactionTask.unit.spec.ts — simulation failure, post-send failure, bigint payload safety
• SolanaJitoWaitForTransactionTask.unit.spec.ts — bundle failure, bigint regression coverage

Linear

EMB-390