fix(installer): cleanup orphan testgen+postgres and log-file encoding on Windows

[aarthy-dk]

Summary

Two Windows-installer robustness fixes surfaced while testing TG-1083.

1. Orphan cleanup before install/delete. When a previous standalone session exited dirty (force-killed via Task Manager, browser tab close, etc.), the embedded postgres survives — it was spawned with `CREATE_NEW_PROCESS_GROUP | CREATE_NO_WINDOW`. Next `tg install` then fails at `standalone-setup` because the orphan still owns `~/.testgen/pgdata`; next `tg delete` half-finishes because Windows file-locks the running `testgen.exe` binary, blocking `uv tool uninstall`. New `stop_standalone_orphans()` helper kills the postgres PID read from `postmaster.pid` (precise — other Postgres installs untouched) and force-kills `testgen.exe` by image name (safe — installer is `dk-installer.exe`). Wired into `_delete_pip` and `TestgenStandaloneSetupStep.pre_execute`. The pre-execute hook runs only after `_resolve_install_mode` confirms no install marker, so the existing "you already have an install" guard is preserved.
2. UTF-8 log file. `logging.FileHandler` was being constructed without an `encoding=` argument, so on Windows it opened the log file with cp1252 (locale default). Lines containing `✓` (used in prereq-status output) hit UnicodeEncodeError on emit, producing alarming `--- Logging error ---` tracebacks on stderr even though the install completed. Adds `"encoding": "utf-8"` to the dictConfig.

Jira

TG-1084

Test plan

• Full suite still passes locally (182 tests).
• `ruff check` + `ruff format --check` clean.
• Helper is a silent no-op on a clean dev machine.
• Windows: with an orphan `postgres.exe` from a prior dirty exit, `tg install` completes (the failing case from `chip_installer_logs.txt`).
• Windows: with an orphan `testgen.exe`, `tg delete` completes including binary removal.
• Windows: install log file `installer_log.txt` contains `✓` characters without the `--- Logging error ---` traceback (the failing case from `chip_install.txt`).

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
dk-installer.py	1803	425	76%	155–156, 160–161, 173–185, 213–214, 217, 222–226, 236–237, 246–258, 263, 276, 322–323, 382, 422–429, 436, 448, 470–471, 515, 565–568, 587–588, 590, 640–669, 673–710, 741–759, 830, 901–902, 970, 987–988, 1005–1010, 1084–1085, 1095, 1111–1115, 1118, 1121, 1124, 1135–1168, 1271, 1277, 1323, 1326, 1333–1334, 1340–1342, 1442–1453, 1502–1517, 1520–1531, 1548–1549, 1586–1587, 1613–1614, 1621, 1626, 1632–1633, 1666–1669, 1678–1679, 1684–1685, 1701–1712, 1757–1759, 1938–1963, 2004–2007, 2072–2075, 2128, 2153–2155, 2163–2165, 2214, 2216, 2218, 2330–2331, 2349, 2379, 2388, 2391–2393, 2436–2448, 2453–2460, 2468–2478, 2539–2552, 2557–2558, 2576–2577, 2617–2618, 2632, 2678, 2790–2860, 2863–2865, 2868–2870, 2941, 2984–3002, 3006, 3011, 3063–3065, 3068–3070, 3096–3099, 3118–3134, 3137–3139, 3142–3144, 3168–3173, 3189–3191, 3227–3235, 3238–3240, 3248–3252, 3316–3318, 3325–3327, 3338–3341, 3346–3347, 3353–3354, 3365–3374, 3383–3437, 3441–3477, 3481–3496
TOTAL	1803	425	76%

Tests	Skipped	Failures	Errors	Time
182	0 💤	0 ❌	0 🔥	2.477s ⏱️