[108] ISSUE-081: Add Soroban Transaction Preflight Example

[esthertitilayo-dev]

Summary

• Adds src/examples/81-transaction-preflight.ts, demonstrating the full Soroban preflight workflow: connect to RPC, fund an ephemeral fee-payer, build an invocation, run preflight simulation, extract the ledger footprint/authorization entries/resource fee, assemble the final transaction, then sign, submit, and poll for confirmation.
• Unlike 68-soroban-contract-simulation (which stops after assembly), this example completes the lifecycle and explicitly explains how a full preflight differs from an ordinary read-only simulation.
• Preflight failures are handled gracefully via a pure, testable describePreflightFailure helper that never throws and always returns actionable guidance.
• Registers the example in src/runner/catalog.ts and documents it in README.md (roadmap entry + Direct Run section).
• Adds tests/transaction-preflight.test.ts covering footprint extraction (against a real constructed xdr.SorobanTransactionData), auth summarization, resource fee formatting, failure guidance, the preflight-vs-simulation explanation, runner registration, and README docs.
• Adds a validation.config.json exclusion entry for the new example (requires live Soroban RPC + deployed contract).
• Also fixes a pre-existing JSON syntax bug in src/validation/validation.config.json (a missing }, / { between the 67-soroban-contract-events and 50-asset-issuer-discovery exclusion entries) that was breaking tests/ledger-effects.test.ts and the full test suite.

Test plan

• npx eslint "src/**/*.ts" "tests/**/*.ts" clean on all changed/added files
• npx tsc --noEmit — no new errors introduced (pre-existing unrelated errors in 72/73/75 example files remain, untouched by this PR)
• npx jest tests/transaction-preflight.test.ts tests/ledger-effects.test.ts tests/runner.test.ts — all passing
• npm test — full suite green (22 suites, 261 tests)

Closes #108

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