Restore the missing matched text derivation regression from PR 133

[shouze]

Parent epic: #146

Context and problem

Pull request 133 introduced a helper to derive the matched segment text from the fragment and its indices when GitHub omits the text field in a text match segment. The current code in src/api.ts still defaults to an empty string in that case, suggesting this fix was lost or never merged on this branch.

Solution

• Reintroduce the segment text derivation helper in src/api.ts and use it in the mapping performed by fetchAllResults().
• Add the corresponding regression test.

Acceptance criteria

• A test in src/api.test.ts simulates a text match segment without a text field and asserts the derived value equals the fragment sliced at the given indices.
• The matched text appears correctly in the JSON output for this case, covered in src/output.test.ts.

Definition of done

• bun test passes, including the new regression test
• bun run lint, bun run format:check, bun run knip pass
• bun run build.ts compiles

Files

• src/api.ts
• src/api.test.ts