test: add unit tests for Freighter error mapping

[Queen-T16]

Summary

Closes #800 — [Testing] Add unit tests for freighterErrors mapping.

src/utils/freighterErrors.ts previously had no dedicated test coverage. This PR adds a full unit-test suite and extends the module (in the spirit of the already-tested contractErrors.ts) so the required cases are both testable and mapped to user-facing messages.

Changes

src/utils/freighterErrors.ts

• New isExtensionNotInstalled(error) — detects the "Freighter extension not installed / unavailable" case (patterns like not installed, is not defined, window.freighter, no freighter).
• New FreighterExtensionNotInstalledError — typed error thrown by wrapFreighterError() for the missing-extension case.
• New parseFreighterError(error): string — maps any thrown value to a user-facing message:
  • user rejection → USER_CANCELLED_MESSAGE
  • missing extension → FREIGHTER_NOT_INSTALLED_MESSAGE
  • readable unrecognized errors → raw message
  • unrecognized error objects (e.g. {}, { code: 4001 }), null, undefined → GENERIC_FREIGHTER_ERROR_MESSAGE (no crash)
• Message strings centralized as exported constants; a shared errorMessage() extractor handles Error, strings, flat objects, and nested { error: { message } } shapes.
• wrapFreighterError() now re-throws typed errors for both known cases and re-throws unrecognized errors unchanged (existing callers in contractClient.ts / offchainApiClient.ts are unaffected).

src/__tests__/utils/freighterErrors.test.ts (new)

• User-rejected signature case across formats (Error / string / object / nested object).
• Extension-not-installed case across formats, plus a guard that unrelated errors are not misclassified.
• Unrecognized error objects falling back to a generic message rather than crashing.

Note: placed at src/__tests__/utils/freighterErrors.test.ts (the repo's existing convention, e.g. exportCsv.test.ts) rather than the literal src/tests/ path from the issue, since the repo has no src/tests directory and Jest's config ignores only the root tests/ e2e dir.

Acceptance criteria

• Add src/tests/utils/freighterErrors.test.ts (see note above — added under src/__tests__/utils/ per repo convention).
• Cover the user-rejected-signature case.
• Cover the extension-not-installed case.
• Cover an unrecognized error object falling back to a generic message rather than crashing.

Verification

• New suite: 11 tests pass (npx jest src/__tests__/utils/freighterErrors.test.ts).
• eslint clean on both touched files.
• No changes to how WalletContext surfaces errors (out of scope).

Pre-existing failures unrelated to this change (verified on main): src/__tests__/lib/contractClient.test.ts (1 test) and the typecheck errors in CauseDetailClient.tsx / WalletContext.tsx.

Out of scope

Changing how WalletContext surfaces these errors in the UI (explicitly out of scope in the issue).

[sshdopey]

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[davidmaronio]

solid coverage: the new errorMessage extractor handles nested { error: { message } } shapes, isExtensionNotInstalled fills a real gap, and the tests exercise rejection, missing-extension, unrecognized objects, and the wrap/rethrow contract. one small note, non-blocking:

1. src/utils/freighterErrors.ts:11: the new user-facing strings (FREIGHTER_NOT_INSTALLED_MESSAGE, GENERIC_FREIGHTER_ERROR_MESSAGE) are hardcoded english, while contractErrors.ts maps to i18n keys resolved via messages/*.json. following that convention would keep these translatable; fine as a follow-up if the maintainers prefer.

gate: branch is behind main; rebase and rerun CI, then good to merge. (no stacking concerns with your #1023/#1025, the files are disjoint.)