Fix cookie-jar pollution; remove dead QR login code; add tests

The reauth flow ran validate_jsessionid against HA's shared aiohttp
session, then update_cookies({"JSESSIONID": ...}) added the new value
with no domain. aiohttp's cookie jar still held the previous
JSESSIONID pinned to smartthingsfind.samsung.com (set via Set-Cookie
during the previous load), and aiohttp prefers the more-specific
domain match — so the *stale* cookie went out, Samsung returned no
_csrf header, and the user saw "Cookie was rejected by SmartThings
Find" even though their cookie was fine.

Two fixes:

* validate_jsessionid now runs in an isolated aiohttp.ClientSession
  with its own jar, so the shared HA jar can't shadow the cookie
  under test.

* async_setup_entry clear_domain()s the smartthingsfind.samsung.com
  cookies before reseating JSESSIONID with response_url, otherwise
  the same shadowing breaks the entry reload that follows a
  successful UI reauth.

Also remove the QR-code login code (do_login_stage_one / _two,
gen_qr_code_base64, the legacy URL constants and qrcode/base64/
random/string/re/asyncio/io/timedelta imports) — Samsung migrated
account.samsung.com to a SPA-driven IAM/OAuth2 flow months ago, so
the QR scrape no longer works and nothing in the integration
references those helpers anymore. Drops the qrcode/pillow/requests
manifest requirements.

Tests: a minimal conftest stubs the homeassistant.* imports the
integration uses, and four async tests cover validate_jsessionid
including the regression case where a domain-bound stale cookie
sits in the shared jar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

custom_components/smartthings_find/manifest.json

@@ -2,13 +2,13 @@
   "domain": "smartthings_find",
   "name": "SmartThings Find",
   "after_dependencies": ["http"],
-  "version": "0.3.0",
+  "version": "0.3.1",
   "documentation": "https://git.jeena.net/jeena/HA-SmartThings-Find",
   "issue_tracker": "https://git.jeena.net/jeena/HA-SmartThings-Find/issues",
   "integration_type": "hub",
   "dependencies": [],
   "codeowners": ["@jeena"],
-  "requirements": ["requests", "qrcode[pil]", "pillow", "pytz"],
+  "requirements": ["pytz"],
   "iot_class": "cloud_polling",
   "config_flow": true
 }