fix(mcp): classify paid call errors by structure and codes before text matching (#410)

[sparkysolutions10-pixel]

Resolves #410

Summary of Changes

This PR refactors handlePaidCallError in src/mcp/server.ts to classify failures by error structure, Horizon result codes, and HTTP statuses first, rather than relying on brittle message substring inspection:

1. Structure-First Error Classification (Errors are classified by searching the message text #410):

   • Horizon Result Codes: Inspects e.response?.data?.extras?.result_codes and e.resultCodes:
     • op_no_trust maps directly to ERR_MISSING_TRUSTLINE
     • op_underfunded, op_low_reserve, tx_insufficient_balance, tx_insufficient_fee map directly to ERR_INSUFFICIENT_BALANCE
     • tx_bad_auth, tx_bad_seq, tx_too_late, op_bad_auth map to ERR_EXPIRED_AUTHORISATION
   • Explicit Ledger Expiration: Inspects signatureExpirationLedger and currentLedger properties or error code fields.
   • HTTP Status Codes: Maps x402 HTTP client status codes directly:
     • 402 -> ERR_PAYMENT_REJECTED
     • 401/403 -> ERR_EXPIRED_AUTHORISATION
     • 502/503/504 -> ERR_SERVICE_UNREACHABLE
     • 400-range -> ERR_BAD_REQUEST
   • Network & Socket Codes: Inspects e.code / e.cause.code (ENOTFOUND, ECONNREFUSED, ETIMEDOUT) and timeout names.
   • Anti-Fragile Fallback: Text matching (msg.includes(...)) is retained purely as a last-resort fallback for unstructured third-party errors. Messages containing the word "fetch" with underlying trustline errors are no longer misclassified as service outages.

2. Vitest Unit Test Suite:

   • Added tests in tests/mcp.test.ts verifying structure-first discrimination across Horizon operations, transaction codes, HTTP statuses, and anti-fragility against reworded messages.
   • All 12 tests passing in 37ms.
   • Changeset added under .changeset/mcp-structured-error-classification.md.

[Miracle656]

Changes requested: one issue per PR, and only issues assigned to you

This is a process requirement, applied to all 23 of your open PRs in this repo. It is not a judgement about the code, which I have not been able to review — and that is the first half of the problem.

The bundling

Across your open PRs, 150 distinct issues are claimed. This one claims several. #483 claims 22, #481 claims 20, #477 claims 20, #474 claims 20.

A PR that closes twenty issues cannot be reviewed. Each of those issues has its own acceptance criteria — typically five — so twenty issues is a hundred separate things to verify in one diff, and if any one of them is wrong the whole PR is blocked while it is fixed. Every batch I published says the issues are deliberately independent so they can be worked in parallel, by different people, without colliding. Bundling them removes exactly that property.

The assignments

135 of the 150 issues claimed are assigned to other contributors — 67 different people.

This PR is an example: it names issues currently assigned to other contributors who are working on them now. Several have open PRs of their own. If this merged as it stands, those people would have done the work and received nothing for it, and I would have no way to tell the two efforts apart.

I am not assuming anything about why. Assignments change, Drips and GitHub can disagree, and a wide-ranging branch picks up issue numbers easily. But the effect on the people holding those assignments is the same regardless of intent, which is why I have to act on it.

What I need

• One issue per PR. The issue number in the title, its acceptance criteria met, and nothing else in the diff.
• Only issues assigned to you, or unassigned ones you have claimed through Drips. If an issue is assigned to someone else, it is theirs.
• Re-open the work that meets both conditions as individual PRs. I will review them in the order they arrive and at the same pace as everyone else's.

Nothing is being closed. These stay open and are yours to re-cut, withdraw or split as you choose.

Two things worth knowing before you re-cut

Some of this has already landed. #488 (SSRF guard for bazaar_search) and #487 (the paid_call host allow-list) cover the same ground as your #449; #486 covers the MCP README. Check current main before rebuilding something that is already there.

main has moved a long way today — #306, #309, #252, #256, #355, #381, #390, #402, #476, #488 and more. Cut from current main, not from where these branches started.

If you believe an issue named here is genuinely yours, say which and I will check the assignment myself — I would rather be corrected than have someone lose work.