fix: add isPendingTransactionStatus helper

[shoaib050326]

What does this PR do?

Adds a public isPendingTransactionStatus(status: TransactionStatus): boolean helper for SEP-24 transaction statuses.

The helper is exported from the public types surface and documents the exact statuses it treats as pending or in-progress:

• pending_anchor
• pending_user_transfer_start
• pending_user_transfer_complete
• pending_external
• pending_trust
• pending_user
• pending_stellar

How to test?

bun install --frozen-lockfile
bun run lint
bun test

Checklist

• My code follows the code style of this project.
• I have added tests for my changes.
• I have updated the documentation accordingly.
• I have run bun run test and bun run lint locally.

Issue Reference

Closes #132

[gemini-code-assist]

Code Review

This pull request introduces a utility function isPendingTransactionStatus and a set of pending transaction statuses, supported by new unit tests. The review feedback recommends enhancing the implementation by defining a PendingTransactionStatus type and utilizing it as a TypeScript type guard to facilitate type narrowing in downstream logic.

[shoaib050326]

Resolved medium Priority Issue.