ci(windows): make --single-process Chromium darwin-only in playwright tests

Chat UI Tests on windows-latest were dying at composer.wait_for(...)
with playwright TargetClosedError "Locator.wait_for: Target page,
context or browser has been closed". studio.log shows a clean POST
/api/auth/change-password 200 followed by zero further requests --
the page died as soon as the React app navigated after the
change-password submit. The root cause is the --single-process
Chromium flag in _CHROMIUM_STABILITY_ARGS: it was added in commit
fdf7f94f for the macos-14 free runner, where the browser <-> renderer
IPC pipe was the actual crash site, but on windows-latest the IPC
pipe is fine and forcing single-process strictly destabilises the
browser -- any in-flight renderer crash takes the whole context
down because there is no separate renderer process to recover into.

Make the flag conditional on sys.platform == "darwin" in both
playwright_chat_ui.py and playwright_extra_ui.py. Linux currently
passes either way today, so we mirror the original commit's stated
intent ("ci(mac): single-process Chromium") and only opt darwin in.
The accompanying timeout / screenshot-best-effort comments stay
correct -- they describe darwin-specific slowness that is still
real on the macos-14 runner.

Failing run for the record: 25522501202 / job 74909947457.

tests/studio/playwright_chat_ui.py

@@ -132,8 +132,17 @@ with sync_playwright() as p:
         "--disable-dev-shm-usage",
         "--no-sandbox",
         "--disable-gpu",
-        "--single-process",
     ]
+    # --single-process is a macos-14 free-runner workaround. On
+    # windows-latest it is strictly destabilising: any renderer
+    # crash (including the React redirect after change-password)
+    # takes the entire browser context down, surfacing as
+    # TargetClosedError on the next Locator.wait_for. Run
+    # 25522501202 / job 74909947457 had the page die right after
+    # POST /api/auth/change-password 200 with this flag enabled.
+    # Linux passes either way today, so we only opt in on darwin.
+    if sys.platform == "darwin":
+        _CHROMIUM_STABILITY_ARGS.append("--single-process")
     browser = p.chromium.launch(
         headless = True,
         args = _CHROMIUM_STABILITY_ARGS,

tests/studio/playwright_extra_ui.py

@@ -89,8 +89,14 @@ with sync_playwright() as p:
         "--disable-dev-shm-usage",
         "--no-sandbox",
         "--disable-gpu",
-        "--single-process",
     ]
+    # --single-process is darwin-only -- on windows-latest it
+    # collapses the renderer-isolation safety net and any in-flight
+    # crash (e.g. React redirect after change-password) takes the
+    # whole browser context down. Same rationale as
+    # playwright_chat_ui.py.
+    if sys.platform == "darwin":
+        _CHROMIUM_STABILITY_ARGS.append("--single-process")
     browser = p.chromium.launch(
         headless = True,
         args = _CHROMIUM_STABILITY_ARGS,