fix: fill thesis grade only in last semester and fix finish() redirect

[ffooll-bit]

Summary

Fixes two blocking defects in the PISN graduation wizard's submission path plus the thesis-grade fill target:

1. finish() fatal TypeError on submit — Graduation::finish() was declared : string but returns a RedirectResponse, throwing TypeError: Return value must be of type string the moment the wizard is actually submitted to Neo Feeder. Changed the return type to CodeIgniter\HTTP\RedirectResponse.

2. finish() time limit exceeded — the full submission iterates the whole batch with a synchronous Neo Feeder call chain per student (InsertMahasiswaLulusDO + academic edits + thesis grade), which could exceed PHP's default 60 s limit on large batches. Added set_time_limit(0) at the top of finish().

3. Thesis grade fill targeted the wrong semester (ENH-010 regression) — when a student had several thesis/skripsi rows across semesters (e.g. retakes), card 3 tagged every missing-grade row "(akan diisi)" and finish() could push the Excel grade into an older row. Now only the thesis/skripsi in the last semester (id_smt largest) is targeted, and only while that row is still ungraded; if the last-semester thesis already has a grade, no value is filled and no older row is backfilled.

Fixes

#113

Checklist

• php -l clean on changed files
• php-cs-fixer fix --dry-run — 0 files
• PHPUnit — 36/36 tests pass
• Logic check (temp) — 4 cases pass (201910176 → 20252; 201910085 → null; graded-only → null)
• Playwright live — NIM 201910085 shows 0 "(akan diisi)" (last semester already graded, nothing backfilled); NIM 201910176 tags only 20252

Notes

• docs/ARCHITECTURE.md / docs/STRUCTURE.md left unstaged (out of scope).
• Syntax/CS/unit verified locally; the live submission path (TypeError + time limit) is confirmed by the reporter's own Neo Feeder push attempt after these fixes.